fix PR7953 - Windows filename are case insensitive:

#pragma once wasn't working on win32 if the header file was included
using a different case.
I tracked down  the problem to the fact that clang::FileManager was
caching files using case sensitive string (UniqueFiles) on Windows.

I changed FileManager to cache filename in lower case only.
Doesn't affect UNIX because UNIX uses Inode to uniquely identify files.

unix doesn't use this codepath.

Analysis and patch by Francois Pichet!

llvm-svn: 111866