In openFileForRead, attempt to fetch the actual name of the file on disk --...

In openFileForRead, attempt to fetch the actual name of the file on disk -- including case -- so that clang can later warn about non-portable #include and #import directives.

Differential Revision: http://reviews.llvm.org/D19842
Corresponding clang patch: http://reviews.llvm.org/D19843

Re-commit after addressing issues with of generating too many warnings for Windows and asan test failures

Patch by Eric Niebler

llvm-svn: 272555