[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

libcxx/test/std/utilities/function.objects/refwrap/type_properties.pass.cpp

@@ -15,7 +15,7 @@
 // CopyAssignable, and TriviallyCopyable (starting in C++14).
 
 // Test fails due to use of is_trivially_* trait.
-// XFAIL: gcc-4.9
+// XFAIL: gcc-4.9 && c++14
 
 #include <functional>
 #include <type_traits>