Revert "revert r179735, it has no testcases, and doesn't really make sense."

This un-reverts r179735 and reverts commit r180574.

This fixes assertion failures for me locally and should fix the failures
on Windows reported widely on llvm-dev.  We should check if the bots
caught this and if so why not.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180722 91177308-0d34-0410-b5e6-96231b3b80d8