Remove symtab links from dwarf2_psymtab and dwarf2_per_cu_quick_data

Remove symtab links from dwarf2_psymtab and dwarf2_per_cu_quick_data

The dwarf2_psymtab and dwarf2_per_cu_quick_data types contain a pointer
to a compunit_symtab, which is a pointer to the corresponding full
symtab.  The dwarf2_psymtab and dwarf2_per_cu_quick_data objects are
going to become objfile-independent, and possibly shared by multiple
objfiles, whereas compunit_symtab will stay objfile-dependent.  This
backlink to the compunit_symtab must therefore be removed.

This patch replaces them with a vector in the dwarf2_per_objfile type,
that serves as a mapping from dwarf2_per_cu_data objects to
compunit_symtab objects, for this particular objfile.  The vector is
indexed using the index assigned to the dwarf2_per_cu_data at its
creation.

I removed the get_compunit_symtab, as it appears to bring not much value
over calling dwarf2_per_objfile::get_symtab directly.

gdb/ChangeLog:

YYYY-MM-DD  Tom Tromey  <tom@tromey.com>
YYYY-MM-DD  Simon Marchi  <simon.marchi@efficios.com>

	* dwarf2/read.h (struct dwarf2_per_bfd) <num_psymtabs>: New
	method.
	(struct dwarf2_per_objfile) <resize_symtabs, symtab_set_p,
	get_symtab, set_symtab>: New methods.
	<m_symtabs>: New field.
	(struct dwarf2_psymtab): Derive from partial_symtab.
	<readin_p, get_compunit_symtab>: Declare methods.
	* dwarf2/read.c (dwarf2_per_objfile::symtab_set_p,
	dwarf2_per_objfile::get_symtab, dwarf2_per_objfile::set_symtab):
	New methods.
	(struct dwarf2_per_cu_quick_data) <compunit_symtab>: Remove.
	(dw2_do_instantiate_symtab, dw2_instantiate_symtab)
	(dw2_map_expand_apply, dw2_map_symtabs_matching_filename)
	(dw2_symtab_iter_next, dw2_print_stats)
	(dw2_expand_symtabs_with_fullname)
	(dw2_expand_symtabs_matching_one)
	(dw_expand_symtabs_matching_file_matcher)
	(dw2_find_pc_sect_compunit_symtab, dw2_map_symbol_filenames)
	(dw2_debug_names_iterator::next)
	(dw2_debug_names_map_matching_symbols)
	(fill_in_sig_entry_from_dwo_entry, dwarf2_psymtab::read_symtab)
	(process_queue, dwarf2_psymtab::expand_psymtab): Update.
	(dwarf2_psymtab::readin_p, dwarf2_psymtab::get_compunit_symtab):
	New methods.
	(get_compunit_symtab, process_full_comp_unit)
	(process_full_type_unit): Update.
	(dwarf2_build_psymtabs, dwarf2_initialize_objfile, add_type_unit): Call

Change-Id: Iec53d96e0b70a57d8b68408febdac3c6c3d4854b