[stdlib] add no-parameter initializers for Span types

This was an addition in SE-0485.

Author: glessard

stdlib/public/core/Span/MutableRawSpan.swift

@@ -32,6 +32,14 @@ public struct MutableRawSpan: ~Copyable & ~Escapable {
     unsafe _pointer._unsafelyUnwrappedUnchecked
   }
 
+  @_alwaysEmitIntoClient
+  @inline(__always)
+  @lifetime(immortal)
+  public init() {
+    unsafe _pointer = nil
+    _count = 0
+  }
+
   @unsafe
   @_unsafeNonescapableResult
   @_alwaysEmitIntoClient

stdlib/public/core/Span/MutableSpan.swift

@@ -33,6 +33,14 @@ public struct MutableSpan<Element: ~Copyable>
     unsafe _pointer._unsafelyUnwrappedUnchecked
   }
 
+  @_alwaysEmitIntoClient
+  @inline(__always)
+  @lifetime(immortal)
+  public init() {
+    unsafe _pointer = nil
+    _count = 0
+  }
+
   @unsafe
   @_unsafeNonescapableResult
   @_alwaysEmitIntoClient

stdlib/public/core/Span/RawSpan.swift

@@ -54,7 +54,7 @@ public struct RawSpan: ~Escapable, Copyable, BitwiseCopyable {
   @_alwaysEmitIntoClient
   @inline(__always)
   @lifetime(immortal)
-  internal init() {
+  public init() {
     unsafe _pointer = nil
     _count = 0
   }

stdlib/public/core/Span/Span.swift

@@ -55,7 +55,7 @@ public struct Span<Element: ~Copyable>: ~Escapable, Copyable, BitwiseCopyable {
   @_alwaysEmitIntoClient
   @inline(__always)
   @lifetime(immortal)
-  internal init() {
+  public init() {
     unsafe _pointer = nil
     _count = 0
   }