Currently we can only remap a file by creating a MemoryBuffer and replacing the file...

Currently we can only remap a file by creating a MemoryBuffer and replacing the file contents with it.
Allow remapping a file by specifying another filename whose contents should be loaded if the original
file gets loaded. This allows to override files without having to create & load buffers in advance.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127052 91177308-0d34-0410-b5e6-96231b3b80d8