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

[Ehsan Akhgari]

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.  :-)

http://reviews.llvm.org/D4201