Use the name of the file on disk to issue a new diagnostic about non-portable #include and #import paths.

Differential Revision: http://reviews.llvm.org/D19843
Corresponding LLVM change: http://reviews.llvm.org/D19842

Re-commit of r272562 after addressing clang-x86-win2008-selfhost failure.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272584 91177308-0d34-0410-b5e6-96231b3b80d8