Concurrency: Implement high-level withUnsafe[Throwing]Continuation entry points

These functions call the corresponding built-ins,
wrapping the Builtin.RawUnsafeContinuation value
in an Unsafe[Throwing]Continuation<T>.

Author: slavapestov

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -22,25 +22,34 @@ public struct PartialAsyncTask {
 
 @frozen
 public struct UnsafeContinuation<T> {
-  private var context: Builtin.RawUnsafeContinuation
+  @usableFromInline internal var context: Builtin.RawUnsafeContinuation
 
   public func resume(returning: __owned T) { }
+
+  @_alwaysEmitIntoClient
+  internal init(_ context: Builtin.RawUnsafeContinuation) {
+    self.context = context
+  }
 }
 
 @frozen
 public struct UnsafeThrowingContinuation<T> {
-  private var context: Builtin.RawUnsafeContinuation
+  @usableFromInline internal var context: Builtin.RawUnsafeContinuation
 
   public func resume(returning: __owned T) { }
   public func resume(throwing: __owned Error) { }
+
+  @_alwaysEmitIntoClient
+  internal init(_ context: Builtin.RawUnsafeContinuation) {
+    self.context = context
+  }
 }
 
 #if _runtime(_ObjC)
 
 // Intrinsics used by SILGen to resume or fail continuations
 // for
 @_alwaysEmitIntoClient
-@usableFromInline
 internal func _resumeUnsafeContinuation<T>(
   _ continuation: UnsafeContinuation<T>,
   _ value: __owned T
@@ -49,7 +58,6 @@ internal func _resumeUnsafeContinuation<T>(
 }
 
 @_alwaysEmitIntoClient
-@usableFromInline
 internal func _resumeUnsafeThrowingContinuation<T>(
   _ continuation: UnsafeThrowingContinuation<T>,
   _ value: __owned T
@@ -58,7 +66,6 @@ internal func _resumeUnsafeThrowingContinuation<T>(
 }
 
 @_alwaysEmitIntoClient
-@usableFromInline
 internal func _resumeUnsafeThrowingContinuationWithError<T>(
   _ continuation: UnsafeThrowingContinuation<T>,
   _ error: __owned Error
@@ -67,3 +74,22 @@ internal func _resumeUnsafeThrowingContinuationWithError<T>(
 }
 
 #endif
+
+// Wrappers around unsafe continuation builtins
+@_alwaysEmitIntoClient
+public func withUnsafeContinuation<T>(
+  _ fn: (UnsafeContinuation<T>) -> Void
+) async -> T {
+  return await Builtin.withUnsafeContinuation {
+    fn(UnsafeContinuation<T>($0))
+  }
+}
+
+@_alwaysEmitIntoClient
+public func withUnsafeThrowingContinuation<T>(
+  _ fn: (UnsafeThrowingContinuation<T>) -> Void
+) async throws -> T {
+  return try await Builtin.withUnsafeThrowingContinuation {
+    fn(UnsafeThrowingContinuation<T>($0))
+  }
+}