Replace r213816's fix with a different one. It's not meaningful to call

isOnePastTheEnd on an invalid designator, so assert and push the check into the
one caller that wasn't already checking.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213820 91177308-0d34-0410-b5e6-96231b3b80d8