Remove incorrect #ifdef guards around variant tests.

The tests were previously guarded by #if defined(_LIBCPP_VER) || defined(_MSVC_STL_VER),
which is both incorrect (e.g. _LIBCPP_VERSION) and unneeded. Although the tests are
technically non-standard (yet) they are supported by both libc++ and MSVC's STL.

libstdc++ doesn't regularly use the test suite so I'm not concerned about guarding these
tests for them.

llvm-svn: 303953