Recommit r296712: "Fix Apple-specific XFAIL directive in libc++ test"

The test is passing with c++11 and c++14 but not c++1z on this
particular version of the compiler. Try to use lit boolean condition
to satisfy this constaint.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@296725 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed