PR19966: don't crash/assert when __underlying_type is applied to an incomplete
enumeration type. I've also filed a LWG issue pointing out that this should be
ill-formed.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210723 91177308-0d34-0410-b5e6-96231b3b80d8