Heavily based on a patch from
Aaron Wishnick <aaron.s.wishnick@gmail.com>.

I'll clean up the duplicated function in CodeGen as
a follow-up, later today or tomorrow.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159060 91177308-0d34-0410-b5e6-96231b3b80d8