Permit -Wswitch coverage for enumerations.

Thanks to David Blakie and Richard Smith for pointing out that we can retain the
-Wswitch coverage while avoiding the warning from GCC by pushing the unreachable
outside of the switch!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210812 91177308-0d34-0410-b5e6-96231b3b80d8