Adding HardLink Support to VirtualFileSystem.

Summary:
Added support of creating a hardlink from one file to another file.
After a hardlink is added between two files, both file will have the same:
  1. UniqueID (inode)
  2. Size
  3. Buffer

This will bring replay of compilation closer to the actual compilation. There are instances where clang checks for the UniqueID of the file/header to be loaded which leads to a different behavior during replay as all files have different UniqueIDs.

Patch by Utkarsh Saxena!

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51359

llvm-svn: 341366