Convert dwarf2_per_objfile::die_type_hash to new hash table

Convert dwarf2_per_objfile::die_type_hash, which maps debug info
offsets to `type *`, to gdb::unordered_map.

Change-Id: I5c174af64ee46d38a465008090e812acf03704ec
Approved-By: Tom Tromey <tom@tromey.com>
2 files changed