gdb/dwarf2: rename some things, index -> gdb_index

This renaming helps make it clearer that these entites (classes,
functions) are specific to .gdb_index only, they are not shared with the
.debug_names handling.

Change-Id: I1a3cf3dbf450b62d1a0879d9aedd26397abdfd13
Approved-By: Tom Tromey <tom@tromey.com>