[tests] Add an available feature that combines the triple and use_system_lib.

  - This is so that we can easily write XFAIL markers for tests that are known
    to fail with versions of libc++ as were shipped with a particular triple.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174443 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed