[libc++][test] Fix noexcept assertions in variant's get tests

All constant expressions are non-potentially-throwing in C++14, but that is *not* the case in C++17. Change these tests of the `variant`-flavored overloads of `std::get` to expect the correct behavior when the compiler is not GCC or is GCC 9+.

Credit to Jonathan Wakely for providing an improved version of my initial change that validates the incorrect behavior on GCC < 9 as well as validating the correct behavior on other compilers.

Differential Revision: https://reviews.llvm.org/D61033

llvm-svn: 359220