[libc++] Fix incorrect UNSUPPORTED annotation

The test was marked as UNSUPPORTED for clang-6 and clang-6, instead of
clang-6 and clang-7.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@368666 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed