This is a follow-up to r279112 (which removed the need for the header)
and in the same spirit as r279035 (which attempted to un-break
out-of-tree builds).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279348 91177308-0d34-0410-b5e6-96231b3b80d8