[-fms-extensions] __is_interface_class was miscategorized

We claimed that the __is_interface_class keyword was KEYCXX even though
the __interface keyword was KEYMS.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193065 91177308-0d34-0410-b5e6-96231b3b80d8