Reintroduce a check accidentally removed in 288873 to fix clang bots

Reintroduce a check accidentally removed in 288873 to fix clang bots

I believe this is the cause of the failure, but have not been able to confirm.  Note that this is a speculative fix; I'm still waiting for a full build to finish as I synced and ended up doing a clean build which takes 20+ minutes on my machine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288886 91177308-0d34-0410-b5e6-96231b3b80d8