gdb/dwarf: change a bunch of functions to be methods of cooked_index_worker_debug_info

Move a few functions exclusively used to process units to become methods
of cooked_index_worker_debug_info.  Rename them to a more consistent
name scheme, which gets rid of outdated naming.  The comments were also
quite outdated.

Change-Id: I2e7dcc2e4ff372007dcb4f6c3d34187c9cc2da05
Approved-By: Tom Tromey <tom@tromey.com>