Revert "Fix Apple-specific XFAIL directive in libc++ test"

This reverts commit r296712. It broke our bot.

It turns out that the test is passing with c++11 and c++14 but
not c++1z on this particular version of the compiler. Since one
job is defaulting to c++1z and the other is testing all config I'm
not sure how to fix this...

llvm-svn: 296724