This failure mode shows up occasionally when users try to include C headers in
C++ projects or when porting from Windows. We might as well recover in the way
the user expected, thus avoiding confusing diagnostic messages at point of use.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209963 91177308-0d34-0410-b5e6-96231b3b80d8