[libcxx][test] Make LIBCPP_STATIC_ASSERT usable at namespace scope

... even when `!defined(_LIBCPP_VERSION)`. (Note that the previous definition for this case - `((void)0);` - is ill-formed at namespace scope.) Ditto for `LIBCPP_ASSERT`, `LIBCPP_ASSERT_NOEXCEPT`, `LIBCPP_ASSERT_NOT_NOEXCEPT`, and `LIBCPP_ONLY`.

Differential Revision: https://reviews.llvm.org/D116880