Restore support for dwp v2 DWARF package file format.

Restore support for dwp v2 DWARF package file format.

gold/
	* dwarf_reader.cc (Dwarf_info_reader::visit_type_unit): Add
	tu_length parameter.  Adjust all callers.
	* dwarf_reader.h (Dwarf_info_reader::visit_type_unit): Likewise.
	* dwp.cc: Include dwarf.h.
	(Section_bounds): New struct type.
	(Unit_set): New struct type.
	(Dwo_file::Dwo_file): Initialize new data member.
	(Dwo_file::read_compunit_index, Dwo_file::read_typeunit_index):
	Combine and rename to...
	(Dwo_file::read_unit_index): ...this.
	(Dwo_file::sized_read_compunit_index)
	(Dwo_file::sized_read_typeunit_index): Combine and rename to...
	(Dwo_file::sized_read_unit_index): ...this.
	(Dwo_file::copy_section): Remove section_name, is_str_offsets
	parameters; add section_id parameter.
	(Dwo_file::add_cu_set, Dwo_file::add_tu_set): Combine and rename to...
	(Dwo_file::add_unit_set): ...this.
	(Dwo_file::shndx_map_): Remove.
	(Dwo_file::sect_offsets_): New data member.
	(Dwp_output_file::Dwp_output_file): Initialize new data members.
	(Dwp_output_file::add_section): Rename to...
	(Dwp_output_file::add_contribution): ...this.
	(Dwp_output_file::add_cu_set): Combine parameters into a struct.
	(Dwp_output_file::add_tu_set): Likewise.
	(Dwp_output_file::Contribution): New type.
	(Dwp_output_file::Section::contributions): New data member.
	(Dwp_output_file::Cu_or_tu_set): Remove.
	(Dwp_output_file::Section::Section): New ctor.
	(Dwp_output_file::Dwp_index::Shndx_pool): Remove.
	(Dwp_output_file::Dwp_index::Section_table): New type.
	(Dwp_output_file::Dwp_index::Dwp_index): Initialize new data members.
	(Dwp_output_file::Dwp_index::enter_set): Change type of "set"
	parameter.
	(Dwp_output_file::Dwp_index::shndx_pool): Remove.
	(Dwp_output_file::Dwp_index::shndx_pool_end): Remove.
	(Dwp_output_file::Dwp_index::section_table): New member function.
	(Dwp_output_file::Dwp_index::section_table_end): New member function.
	(Dwp_output_file::Dwp_index::shndx_pool_size): Remove.
	(Dwp_output_file::Dwp_index::section_table_rows): New member function.
	(Dwp_output_file::Dwp_index::section_table_cols): New member function.
	(Dwp_output_file::Dwp_index::shndx_pool_): Remove.
	(Dwp_output_file::Dwp_index::section_table_): New data member.
	(Dwp_output_file::Dwp_index::section_mask_): New data member.
	(Dwp_output_file::add_output_section): New member function.
	(Dwp_output_file::write_new_section): New member function.
	(Dwp_output_file::write_contributions): New member function.
	(Dwp_output_file::section_id_map_): New data member.
	(class Dwo_id_info_reader): Remove.
	(class Unit_reader): New class.
	(get_dwarf_section_name): New function.
	(Dwo_file::read_executable): Adjust initializations of class data.
	(Dwo_file::read): Add support for v2 package file format.
	(Dwo_file::read_unit_index): Likewise.
	(Dwo_file::sized_read_unit_index): Likewise.
	(Dwo_file::copy_section): Likewise.
	(Dwo_file::add_unit_set): Likewise.
	(Dwp_output_file::add_output_section): Likewise.
	(Dwp_output_file::add_contribution): Likewise.
	(Dwp_output_file::Dwp_index::find_or_add): Use row index to check
	for empty slot.
	(Dwp_output_file::Dwp_index::enter_set): Add support for v2 package
	file format.
	(Dwp_output_file::Dwp_index::grow): Use row index to check for empty
	slot.
	(Dwp_output_file::initialize): Remove unused function.
	(Dwp_output_file::finalize): Add support for v2 package file format.
	(Dwp_output_file::write_index): Likewise.
	* gdb-index.cc (Gdb_index_info_reader::visit_type_unit): Adjust
	function prototype.