Fix Apple-specific XFAIL directive in libc++ test

This tests is failing in XCode 7.0. But Xcode 7.3 that shipped
an updated clang has this test passing. This is fixing green dragon
which runs this configuration.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296712 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed