This doesn't appear to be the cause of the slowdown. I'll have to try a
manual bisect to see if there's really anything there, or if it's just
the bot itself taking on additional load. Meanwhile, this change helps
with correctness.

This changes an assertion and adds a test case, then re-applies r180638,
which was reverted in r180714.

<rdar://problem/13296133> and PR15863

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180864 91177308-0d34-0410-b5e6-96231b3b80d8