gdb: rename abbrev_cache to abbrev_table_cache This cache holds `abbrev_table` objects, so I think it's clearer and more consistent to name it `abbrev_table_cache`. Rename it and everything that goes along with it. Change-Id: I43448c0aa538dd2c3ae5efd2f7b3e7b827409d8c Approved-By: Tom Tromey <tom@tromey.com>