Fix Apple-specific XFAIL directive in libc++ test

These tests are failing in XCode 8.0, 8.1, and 8.2, but not in Xcode
8.3. Annoyingly the version numbering for clang does not follow Xcode
and is bumped to 8.1 only in Xcode 8.3. So Xfailing apple-clang-8.0
should catch all cases here.

llvm-svn: 296704