[Support] This sanity check in the test only works with certain versions

of libstdc++, not just certain versions of GCC. The original macros
broke when using Clang + libstdc++4.9 sadly.

Sadly, testing for versions of libstdc++ has been extremely problematic
in the past, so I'm just narrowing this down to Windows and when using
libc++ as that seems at least very unlikely to keep build bots broken.

llvm-svn: 336174