Add tests to ensure that reference_wrapper<T> is trivially copyable. This was added to C++1z with the adoption of N4277, but libc++ already implemented it as a conforming extension. No code changes were needed, just more tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222132 91177308-0d34-0410-b5e6-96231b3b80d8