[PATCH] Fix path concatenation in HeaderSearch

[Yaron Keren]

I fixes the file names shown in clang  messages as

 c:\my\dir/file.cpp

(incosistent slashes) TmpDir arrives in native so always concatenating '/' is wrong. That's what the fixme said...

testcase is tricky as the path seperator changes if clang and the tools were compiled on windows or not. It does not depend on the target but on the host. 

Can we make a test that behave differently based on the host OS?

http://reviews.llvm.org/D3686