PR11778: Fix the rejects-valid half of this bug. We still produce the same
poorly-worded warning for a case value that is not a possible value of the
switched-on expression.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214678 91177308-0d34-0410-b5e6-96231b3b80d8