Consistently use @usableFromInline and remove redundant @inlinables

Author: DougGregor

stdlib/public/core/Array.swift

@@ -1604,7 +1604,6 @@ extension Array {
   ///   for the `withUnsafeBufferPointer(_:)` method. The pointer argument is
   ///   valid only for the duration of the method's execution.
   /// - Returns: The return value, if any, of the `body` closure parameter.
-  @inlinable
   @_alwaysEmitIntoClient
   public func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -1616,11 +1615,8 @@ extension Array {
   // for ABI reasons.
   @_semantics("array.withUnsafeMutableBufferPointer")
   @_effects(notEscaping self.value**)
-  @inlinable // FIXME(inline-always)
-  @inline(__always) // Performance: This method should get inlined into the
-  // caller such that we can combine the partial apply with the apply in this
-  // function saving on allocating a closure context. This becomes unnecessary
-  // once we allocate noescape closures on the stack.
+  @usableFromInline
+  @inline(__always)
   @_silgen_name("$sSa30withUnsafeMutableBufferPointeryqd__qd__SryxGzKXEKlF")
   mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R

stdlib/public/core/ArrayBuffer.swift

@@ -569,7 +569,7 @@ extension _ArrayBuffer {
 
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss12_ArrayBufferV010withUnsafeB7Pointeryqd__qd__SRyxGKXEKlF")
   internal func __abi_withUnsafeBufferPointer<R>(
     _ body: (UnsafeBufferPointer<Element>) throws -> R
@@ -585,7 +585,6 @@ extension _ArrayBuffer {
   /// Call `body(p)`, where `p` is an `UnsafeBufferPointer` over the
   /// underlying contiguous storage.  If no such storage exists, it is
   /// created on-demand.
-  @inlinable
   @_alwaysEmitIntoClient
   internal func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -600,7 +599,7 @@ extension _ArrayBuffer {
 
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss12_ArrayBufferV017withUnsafeMutableB7Pointeryqd__qd__SryxGKXEKlF")
   internal mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (UnsafeMutableBufferPointer<Element>) throws -> R
@@ -612,7 +611,6 @@ extension _ArrayBuffer {
   /// over the underlying contiguous storage.
   ///
   /// - Precondition: Such contiguous storage exists or the buffer is empty.
-  @inlinable
   @_alwaysEmitIntoClient
   internal mutating func withUnsafeMutableBufferPointer<R, E>(
     _ body: (UnsafeMutableBufferPointer<Element>) throws(E) -> R

stdlib/public/core/ArraySlice.swift

@@ -1203,7 +1203,6 @@ extension ArraySlice {
   ///   for the `withUnsafeBufferPointer(_:)` method. The pointer argument is
   ///   valid only for the duration of the method's execution.
   /// - Returns: The return value, if any, of the `body` closure parameter.
-  @inlinable
   @_alwaysEmitIntoClient
   public func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -1214,11 +1213,8 @@ extension ArraySlice {
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
   @_semantics("array.withUnsafeMutableBufferPointer")
-  @inlinable // FIXME(inline-always)
-  @inline(__always) // Performance: This method should get inlined into the
-  // caller such that we can combine the partial apply with the apply in this
-  // function saving on allocating a closure context. This becomes unnecessary
-  // once we allocate noescape closures on the stack.
+  @usableFromInline
+  @inline(__always)
   @_silgen_name("$ss10ArraySliceV30withUnsafeMutableBufferPointeryqd__qd__SryxGzKXEKlF")
   mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R

stdlib/public/core/ContiguousArray.swift

@@ -1144,7 +1144,6 @@ extension ContiguousArray {
   ///   for the `withUnsafeBufferPointer(_:)` method. The pointer argument is
   ///   valid only for the duration of the method's execution.
   /// - Returns: The return value, if any, of the `body` closure parameter.
-  @inlinable
   @_alwaysEmitIntoClient
   public func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -1155,11 +1154,8 @@ extension ContiguousArray {
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
   @_semantics("array.withUnsafeMutableBufferPointer")
-  @inlinable // FIXME(inline-always)
-  @inline(__always) // Performance: This method should get inlined into the
-  // caller such that we can combine the partial apply with the apply in this
-  // function saving on allocating a closure context. This becomes unnecessary
-  // once we allocate noescape closures on the stack.
+  @usableFromInline
+  @inline(__always)
   @_silgen_name("$ss15ContiguousArrayV30withUnsafeMutableBufferPointeryqd__qd__SryxGzKXEKlF")
   mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R

stdlib/public/core/ContiguousArrayBuffer.swift

@@ -446,7 +446,7 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
 
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss22_ContiguousArrayBufferV010withUnsafeC7Pointeryqd__qd__SRyxGKXEKlF")
   internal func __abi_withUnsafeBufferPointer<R>(
     _ body: (UnsafeBufferPointer<Element>) throws -> R
@@ -458,7 +458,6 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
 
   /// Call `body(p)`, where `p` is an `UnsafeBufferPointer` over the
   /// underlying contiguous storage.
-  @inlinable
   @_alwaysEmitIntoClient
   internal func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -470,7 +469,7 @@ internal struct _ContiguousArrayBuffer<Element>: _ArrayBufferProtocol {
 
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss22_ContiguousArrayBufferV017withUnsafeMutableC7Pointeryqd__qd__SryxGKXEKlF")
   internal mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (UnsafeMutableBufferPointer<Element>) throws -> R

stdlib/public/core/SliceBuffer.swift

@@ -432,7 +432,7 @@ internal struct _SliceBuffer<Element>
   //===--- misc -----------------------------------------------------------===//
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss12_SliceBufferV010withUnsafeB7Pointeryqd__qd__SRyxGKXEKlF")
   internal func __abi_withUnsafeBufferPointer<R>(
     _ body: (UnsafeBufferPointer<Element>) throws -> R
@@ -444,7 +444,6 @@ internal struct _SliceBuffer<Element>
 
   /// Call `body(p)`, where `p` is an `UnsafeBufferPointer` over the
   /// underlying contiguous storage.
-  @inlinable
   @_alwaysEmitIntoClient
   internal func withUnsafeBufferPointer<R, E>(
     _ body: (UnsafeBufferPointer<Element>) throws(E) -> R
@@ -456,7 +455,7 @@ internal struct _SliceBuffer<Element>
 
   // Superseded by the typed-throws version of this function, but retained
   // for ABI reasons.
-  @inlinable
+  @usableFromInline
   @_silgen_name("$ss12_SliceBufferV017withUnsafeMutableB7Pointeryqd__qd__SryxGKXEKlF")
   internal mutating func __abi_withUnsafeMutableBufferPointer<R>(
     _ body: (UnsafeMutableBufferPointer<Element>) throws -> R