[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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353206 91177308-0d34-0410-b5e6-96231b3b80d8