Replace dwarf2_per_cu_data::cu backlink with per-objfile map

Replace dwarf2_per_cu_data::cu backlink with per-objfile map

The dwarf2_per_cu_data type is going to become objfile-independent,
while the dwarf2_cu type will stay object-dependent.  This patch removes
the backlink from dwarf2_per_cu_data to dwarf2_cu, in favor of the
dwarf2_per_objfile::m_dwarf2_cus map.  It maps dwarf2_per_cu_data
objects to the corresponding dwarf2_cu objects for this objfile.  If a
CU has been read in in the context of this objfile, then an entry will
be present in the map.

The dwarf2_cu objects that are read in are currently kept in a linked
list rooted in the dwarf2_per_bfd.  Except that the dwarf2_cu objects
are not simply linked together, they are interleaved with their
corresponding dwarf2_per_cu_data objects.  So if we have CUs A and B
read in, the dwarf2_per_bfd::read_in_chain will point to a chain like
this (DPCD == dwarf2_per_cu_data, DC == dwarf2_cu):

 DPCD A -> DC A -> DPCD B -> DC B

Obviously, this can't stay as is, since a same CU can be read in for an
objfile but not read in for another objfile sharing the same BFD, and
the dwarf2_per_cu_data::cu link is removed.   This is all replaced by
the dwarf2_per_objfile::m_dwarf2_cus map.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_cu): Forward-declare.
	(struct dwarf2_per_bfd) <free_cached_comp_units>: Remove,
	move to dwarf2_per_objfile.
	<read_in_chain>: Remove.
	(struct dwarf2_per_objfile) <get_cu, set_cu, remove_cu,
	remove_all_cus, age_comp_units>: New methods.
	<m_dwarf2_cus>: New member.
	(struct dwarf2_per_cu_data) <cu>: Remove.
	* dwarf2/read.c (struct dwarf2_cu) <read_in_chain>: Remove.
	(age_cached_comp_units, free_one_cached_comp_unit): Remove,
	moved to methods of dwarf2_per_objfile.
	(dwarf2_clear_marks): Remove.
	(dwarf2_queue_item::~dwarf2_queue_item): Update.
	(dwarf2_per_bfd::~dwarf2_per_bfd): Don't free dwarf2_cus.
	(dwarf2_per_bfd::free_cached_comp_units): Remove.
	(dwarf2_per_objfile::remove_all_cus): New.
	(class free_cached_comp_units) <~free_cached_comp_units>:
	Update.
	(load_cu): Update.
	(dw2_do_instantiate_symtab): Adjust.
	(fill_in_sig_entry_from_dwo_entry): Adjust.
	(cutu_reader::init_tu_and_read_dwo_dies): Update.
	(cutu_reader::cutu_reader): Likewise.
	(cutu_reader::keep): Use dwarf2_per_objfile::set_cu.
	(cutu_reader::cutu_reader): Use dwarf2_per_objfile::get_cu.
	(process_psymtab_comp_unit): Use dwarf2_per_objfile::remove_cu
	and dwarf2_per_objfile::age_comp_units.
	(load_partial_comp_unit): Update.
	(maybe_queue_comp_unit): Use dwarf2_per_objfile::get_cu.
	(process_queue): Likewise.
	(find_partial_die): Use dwarf2_per_objfile::get_cu instead of cu
	backlink.
	(dwarf2_read_addr_index): Likewise.
	(follow_die_offset): Likewise.
	(dwarf2_fetch_die_loc_sect_off): Likewise.
	(dwarf2_fetch_constant_bytes): Likewise.
	(dwarf2_fetch_die_type_sect_off): Likewise.
	(follow_die_sig_1): Likewise.
	(load_full_type_unit): Likewise.
	(read_signatured_type): Likewise.
	(dwarf2_cu::dwarf2_cu): Don't set cu field.
	(dwarf2_cu::~dwarf2_cu): Remove.
	(dwarf2_per_objfile::get_cu): New.
	(dwarf2_per_objfile::set_cu): New.
	(age_cached_comp_units): Rename to...
	(dwarf2_per_objfile::age_comp_units): ... this.  Adjust
	to std::unordered_map.
	(free_one_cached_comp_unit): Rename to...
	(dwarf2_per_objfile::remove_cu): ... this.  Adjust
	to std::unordered_map.
	(dwarf2_per_objfile::~dwarf2_per_objfile): New.
	(dwarf2_mark_helper): Use dwarf2_per_objfile::get_cu, expect
	a dwarf2_per_objfile in data.
	(dwarf2_mark): Pass dwarf2_per_objfile in data to htab_traverse.
	(dwarf2_clear_marks): Remove.

Change-Id: Ia33ac71c79b2de4710569008e22a6563a1505cde