[PATCH] Refactor the code in clang to find a file in a PATH like environment variable into a helper function

[Hans Wennborg]

> I chose Program.h because I found FindProgramByName there... But I'm not sure if that's the best place either. Can you please suggest either Process.h or FileSystem.h? Either would work for me. :-)

I feel this is tightly related to GetEnv, so let's go for Process.

http://reviews.llvm.org/D4201