stdlib: add SPI for bridging for Foundation

Author: gribozavr

stdlib/public/SDK/Foundation/Foundation.swift

@@ -501,7 +501,7 @@ extension Array : _ObjectiveCBridgeable {
 
   @_semantics("convertToObjectiveC")
   public func _bridgeToObjectiveC() -> NSArray {
-    return unsafeBitCast(self._buffer._asCocoaArray() as AnyObject, to: NSArray.self)
+    return unsafeBitCast(self._bridgeToObjectiveCImpl(), to: NSArray.self)
   }
 
   public static func _forceBridgeFromObjectiveC(

stdlib/public/core/Arrays.swift.gyb

@@ -2091,6 +2091,11 @@ public func != <Element : Equatable>(
 
 #if _runtime(_ObjC)
 extension Array {
+  public // @SPI(Foundation)
+  func _bridgeToObjectiveCImpl() -> AnyObject {
+    return _buffer._asCocoaArray()
+  }
+
   /// Tries to downcast the source `NSArray` as our native buffer type.
   /// If it succeeds, creates a new `Array` around it and returns that.
   /// Returns `nil` otherwise.