Testing the concrete implementation of INVOKE means calling the implementation
specific names `__invoke` and `__invoke_constexpr`. For this reason the test
are non-standard. For this reason it's best if the tests live outside of the
`test/std` directory.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@267973 91177308-0d34-0410-b5e6-96231b3b80d8
