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

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

Patch by Eric Niebler

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271704 91177308-0d34-0410-b5e6-96231b3b80d8