[PATCH] Fix path concatenation in HeaderSearch

[Yaron Keren]

This patch caused one regeression test failure, clang/test/Misc/remap-file.c.

The failure is the result of a FileManager::getVirtualFile limitation: it considers c:\dir\file different than c:/dir/file.
FileManager::getFile may have the same problem but I think it gets sorted out since both names point to same inode and getFile merges them.

Regardless of this patch I think getVirtualFile and getFile should be tolerant to the path seperators.

How to proceed? 
Fix the get*File?
XFAIL the test on Windows and add a Windows only version of it?

http://reviews.llvm.org/D3686