[Foundation] Adjust stray public import of _SwiftFoundationOverlayShims

Author: lorentey

stdlib/public/Darwin/Foundation/NSObject.swift

@@ -12,7 +12,7 @@
 
 @_exported import Foundation // Clang module
 import ObjectiveC
-import _SwiftFoundationOverlayShims
+@_implementationOnly import _SwiftFoundationOverlayShims
 
 // This exists to allow for dynamic dispatch on KVO methods added to NSObject.
 // Extending NSObject with these methods would disallow overrides.