Revert "Remove a pointless assertion."

This reverts commit r177158.

I'm blindly reverting this because it appears to be breaking numerous
buildbots.  I'll reapply if it doesn't turn out to be the culprit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177165 91177308-0d34-0410-b5e6-96231b3b80d8