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.

llvm-svn: 272584
diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index c4cc8dc..ce9b7e1 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -312,6 +312,9 @@
   UFE.InPCH = Data.InPCH;
   UFE.File = std::move(F);
   UFE.IsValid = true;
+  if (UFE.File)
+    if (auto RealPathName = UFE.File->getName())
+      UFE.RealPathName = *RealPathName;
   return &UFE;
 }