Convert dwarf_cu::die_hash to new hash table

Convert one use of htab_t, mapping offsets to die_info object, to
`gdb::unordered_set`.

Change-Id: Ic80df22bda551e2d4c2511d167e057f4d6cd2b3e
Approved-By: Tom Tromey <tom@tromey.com>
4 files changed