[libcxx] Only enable the availability LIT feature when we're testing libc++
[libcxx] Only enable the availability LIT feature when we're testing libc++

Other standard libraries don't implement availability markup, so it doesn't
make sense to e.g. XFAIL tests based on availability markup outside of
libc++.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@348871 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx