Revert r177329.

If this should not happen, we should have an assert.
If it should happen, we should have a test and remove the comment.

In no case should we have this self inconsistent code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177399 91177308-0d34-0410-b5e6-96231b3b80d8