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.
llvm-svn: 267973
The primary purpose of this patch is to add the 'is_callable' traits.
Since 'is_nothrow_callable' required making 'INVOKE' conditionally noexcept
I also took this oppertunity to implement a constexpr version of INVOKE.
This fixes 'std::experimental::apply' which required constexpr 'INVOKE support'.
This patch will be followed up with some cleanup. Primarly removing most
of "__member_function_traits" since it's no longer used by INVOKE (in C++11 at least).
llvm-svn: 266836
