gdb/dwarf: use gdb::unordered_map for dwarf2_per_bfd::{quick_file_names_table,type_un...

gdb/dwarf: use gdb::unordered_map for dwarf2_per_bfd::{quick_file_names_table,type_unit_groups}

Change these two hash tables to use gdb::unordered_map.  I changed these
two at the same time because they both use the same key, a
stmt_list_hash.  Unlike other previous patches that used a
gdb::unordered_set, use an unordered_map here because the key isn't
found in the element itself (well, it was before, because of how htab
works, but it didn't need to be).

You'll notice that the type_unit_group structure is empty.  That
structure isn't really needed.  It is removed in the following patch.

Regression tested on Debian 12 amd64 with a bunch of DWARF target
boards.

Change-Id: Iec2289958d0f755cab8198f5b72ecab48358ba11
Approved-By: Tom Tromey <tom@tromey.com>