Fix -readnow for -gdwarf-4 unused type units.
* dwarf2read.c (struct signatured_type): Remove the field offset.
(create_signatured_type_table_from_index): Remove its initialization.
(create_debug_types_hash_table): Likewise. Initialize per_cu.offset
instead. Add a complaint call.
(process_psymtab_comp_unit): Change assignment to gdb_assert.
(process_type_comp_unit, lookup_die_type, dump_die_shallow)
(lookup_signatured_type_at_offset, read_signatured_type)
(write_one_signatured_type): Update the field for per_cu.
gdb/testsuite/
Fix -readnow for -gdwarf-4 unused type units.
* gdb.dwarf2/dw4-sig-type-unused.S: New file.
* gdb.dwarf2/dw4-sig-type-unused.exp: New file.
