gdb/symtab: Eliminate deferred_entry

gdb/symtab: Eliminate deferred_entry

Currently cooked_index entry creation is either:
- done immediately if the parent_entry is known, or
- deferred if the parent_entry is not yet known, and done later while
  resolving the deferred entries.

Instead, create all cooked_index entries immediately, and keep track of which
entries have a parent_entry that needs resolving later using the new
IS_PARENT_DEFERRED flag.

Tested on x86_64-linux.

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