[libcxx] Fix XFAIL for GCC 4.9

The XFAIL started passing since we're only testing for trivial-copyability of
reference_wrapper in C++14 and above. This commit constrains the XFAIL to
gcc-4.9 with C++14 (it would also fail on C++17 and above, but those standards
are not available with GCC 4.9).

llvm-svn: 347264