Implement (de-)serialization of the buffer contents for an overridden

file in the source manager. This allows us to properly create and use
modules described by module map files without umbrella headers (or
with incompletely umbrella headers). More generally, we can actually
build a PCH file that makes use of  file -> buffer remappings, which
could be useful in libclang in the future.

llvm-svn: 144830