gdb: constification around abbrev_table_cache and abbrev_table

Make `abbrev_table_cache::find` const, make it return a pointer to
`const abbrev_table`, adjust the fallouts.

Make `cooked_index_storage::get_abbrev_table_cache` const, make itreturn
a pointer to const `abbrev_table_cache`.

Change-Id: If63b4b3a4c253f3bd640b13bce4a854eb2d75ece
Approved-By: Tom Tromey <tom@tromey.com>
3 files changed