Try harder to supress the test from r212975 on Windows

The attempt in r212980 was broken because we might not fail if
LLVM_ON_UNIX is enabled for cross compiling to Windows, and it didn't
consider mingw either.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212989 91177308-0d34-0410-b5e6-96231b3b80d8