Initial implementation of virtual file system

This adds the minimum virtual file system support to start migrating
FileManager onto the VFS.

Originally discussed here:
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-February/035188.html

Differential Revision: http://llvm-reviews.chandlerc.com/D2745

llvm-svn: 201618

clang/lib/Serialization/ModuleManager.cpp

@@ -89,7 +89,7 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
     New->InputFilesValidationTimestamp = 0;
     if (New->Kind == MK_Module) {
       std::string TimestampFilename = New->getTimestampFilename();
-      llvm::sys::fs::file_status Status;
+      vfs::Status Status;
       // A cached stat value would be fine as well.
       if (!FileMgr.getNoncachedStatValue(TimestampFilename, Status))
         New->InputFilesValidationTimestamp =