[stdlib] Use _Nonnull over __nonnull in Foundation

The `__nonnull` keyword is deprecated in favor of `_Nonnull`.

Author: modocache

stdlib/public/stubs/FoundationHelpers.mm

@@ -109,6 +109,6 @@ static CFRange cast(_swift_shims_CFRange value) {
 }
 
 _swift_shims_CFStringRef
-swift::_swift_stdlib_objcDebugDescription(id __nonnull nsObject) {
+swift::_swift_stdlib_objcDebugDescription(id _Nonnull nsObject) {
   return [nsObject debugDescription];
 }