Path: Recognize Windows compiled resource file.

Some background: One can pass compiled resource files (.res files) directly
to the linker on Windows. If a resource file is given, the linker will run
"cvtres" command in background to convert the resource file to a COFF file
to link it.

What I'm trying to do with this patch is to make the linker to recognize
the resource file by file magic, so that it can run cvtres command.

Differential Revision: http://llvm-reviews.chandlerc.com/D1943

llvm-svn: 192742