Remove assert added in r201249.

This triggers on one of our internal tests.

Dmitri:
I do not understand this part of the codebase well enough to locate the
underlying cause easily. If the correct fix is not obvious, I can try to
debug the problem further or try to come up with reduced test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201329 91177308-0d34-0410-b5e6-96231b3b80d8