Use unsupported instead of xfail in bytes.pass.cpp upstream libc++ test.

I backported std::byte to work in C++14. Because this
test used XFAIL, the test now unexpectedly passes
instead of fails. I updated to use UNSUPPORTED
instead to avoid an unexpected pass.

libcxx/test/std/language.support/support.types/byte.pass.cpp

@@ -10,7 +10,8 @@
 #include <type_traits>
 #include "test_macros.h"
 
-// XFAIL: c++98, c++03, c++11, c++14
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
 
 // If we're just building the test and not executing it, it should pass.
 // UNSUPPORTED: no_execute