DWARF-5 .debug_names DW_IDX_type_unit fix

DWARF-5 .debug_names DW_IDX_type_unit fix

The .debug_names completely misses its support as it did not even produce
DW_IDX_type_unit.

gdb/ChangeLog
2017-12-14  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* dwarf2read.c (dw2_debug_names_iterator::next): Support
	DW_IDX_type_unit.
	(debug_names::dwarf5_offset_size, unit_kind): New.
	(debug_names::insert): Add parameter kind.
	(debug_names::build): Support DW_IDX_type_unit.
	(debug_names::recursively_write_psymbols): Update
	(debug_names::write_psymbols caller.
	(debug_names::write_one_signatured_type_data)
	(debug_names::write_one_signatured_type): New.
	(debug_names::index_key, debug_names::symbol_value)
	(debug_names::write_psymbols): Add kind.
	(debug_names::write_one_signatured_type): New.
	(write_debug_names): Move dwarf5_offset_size to debug_names.
	Use debug_names::write_one_signatured_type for type units.