[libcxx] Fix the support requirements for `std::function` Objective-C++ test.

[var-const]

Summary

The test requires two features to be supported but only checked for one
of them.

[ldionne]

Thanks, good catch!

libcxx/test/libcxx/utilities/function.objects/func.blocks.arc.pass.mm

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // std::function support for "blocks" when ARC is enabled
 
 // UNSUPPORTED: c++03
 
 // This test requires the Blocks runtime, which is (only?) available on Darwin
 // out-of-the-box.
-// REQUIRES: has-fblocks && darwin
+// REQUIRES: has-fblocks && has-fobjc-arc && darwin
 
 // ADDITIONAL_COMPILE_FLAGS: -fblocks -fobjc-arc
 
 #include <functional>
 
 #include <cassert>
 #include <cstddef>
 #include <string>