Merge pull request #29942 from xymus/spitdlib

[stdlib] Update one String initializer and client to use @_spi

Author: xymus

stdlib/public/Darwin/Foundation/String.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 @_exported import Foundation // Clang module
+@_spi(Foundation) import Swift
 
 //===----------------------------------------------------------------------===//
 // New Strings

stdlib/public/core/StringBridge.swift

@@ -408,8 +408,8 @@ internal func _bridgeCocoaString(_ cocoaString: _CocoaString) -> _StringGuts {
 }
 
 extension String {
-  public // SPI(Foundation)
-  init(_cocoaString: AnyObject) {
+  @_spi(Foundation)
+  public init(_cocoaString: AnyObject) {
     self._guts = _bridgeCocoaString(_cocoaString)
   }
 }