Fix XPASS buildbot failure related to structured bindings

The test was previously set to XFAIL if __cpp_structured_bindings
wasn't defined. However there are Clang 4.0 versions which do not
define this macro but do provide structured bindings, which causes
the test to pass unexpectedly.

This patch changes the XFAIL to an UNSUPPORTED.

llvm-svn: 291060