[libcxx] Make UNSUPPORTED for std::async test more fine grained

The test was previously marked as unsupported on all Apple platforms, when
we really just want to mark it as unsupported for previously shipped dylibs
on macosx.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347920 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed