Sema: Don't crash when trying to emit a warning for a duplicate value in an invalid enum.

Fixes PR15693. A null check on a pointer returned from cast<> is a very dubious
construct, do we have a checker for this somewhere?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178975 91177308-0d34-0410-b5e6-96231b3b80d8