[libc++] [test] C++14/17-friendly `TEST_IS_CONSTANT_EVALUATED` macro.

Reviewed as part of D114658.
Ultimately this will probably have to be flipped around and renamed
`TEST_IS_RUNTIME`, and extended with `TEST_IS_RUNTIME_OR_CXX20` (once
constexpr std::string support is added) and so on for every new C++
version. But we don't need that flexibility yet, so we're not adding it.