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.