gdb/

gdb/
	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.