[Stdlib] Change AnyKeyPath to use @_objcRuntimeName attribute for its rename.

rdar://problem/46546165

Author: mikeash

stdlib/public/core/KeyPath.swift

@@ -29,8 +29,9 @@ internal func _abstract(
 /// A type-erased key path, from any root type to any resulting value
 /// type. NOTE: older runtimes had Swift.AnyKeyPath as the ObjC name.
 /// The two must coexist, so it was renamed. The old name must not be
-/// used in the new runtime.
-@objc(_SwiftAnyKeyPath)
+/// used in the new runtime. _TtCs11_AnyKeyPath is the mangled name for
+/// Swift._AnyKeyPath.
+@_objcRuntimeName(_TtCs11_AnyKeyPath)
 public class AnyKeyPath: Hashable, _AppendKeyPath {
   /// The root type for this key path.
   @inlinable