[libc++] Use UNSUPPORTED instead of TEST_STD_VER #ifdef

When the whole test only works starting at some version of the Standard,
use UNSUPPORTED lit markup instead of #ifdef TEST_STD_VER. This provides
more visibility into the test suite.

Reviewed as https://reviews.llvm.org/D57704.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 353206