Attempt to fix Compiler.h for some self-hosting bots

I tested r185831 by self-hosting clang with a recent clang, and got no
warnings.  I haven't been able to reproduce the problem locally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185833 91177308-0d34-0410-b5e6-96231b3b80d8