Add a flag to FileOutputBuffer that allows modification.

FileOutputBuffer creates a temp file and on commit atomically
renames the temp file to the destination file.  Sometimes we
want to modify an existing file in place, but still have the
atomicity guarantee.  To do this we can initialize the contents
of the temp file from the destination file (if it exists), that
way the resulting FileOutputBuffer can have only selective
bytes modified.  Committing will then atomically replace the
destination file as desired.

llvm-svn: 335902