Replace dw2_get_cu/dw2_get_cutu with methods of dwarf2_per_objfile

Replace dw2_get_cu/dw2_get_cutu with methods of dwarf2_per_objfile

Those two functions look like good candidates to become methods of
dwarf2_per_objfile.  I did that, and added get_tu as well.  When
replacing usages of dw2_get_cutu, I changed some instances to get_cutu
and others to get_cu, when appropriate (when we know we want a CU and
not a TU).

gdb/ChangeLog:

	* dwarf2read.h (struct signatured_type): Forward declare.
	(struct dwarf2_per_objfile) <get_cutu, get_cu, get_tu>:
	New methods.
	* dwarf2read.c (dwarf2_per_objfile::get_cutu): Rename from...
	(dw2_get_cutu): ...this.
	(dwarf2_per_objfile::get_cu): Rename from...
	(dw2_get_cu): ...this.
	(dwarf2_per_objfile::get_tu): New.
	(create_addrmap_from_index): Adjust.
	(create_addrmap_from_aranges): Adjust.
	(dw2_find_last_source_symtab): Adjust.
	(dw2_map_symtabs_matching_filename): Adjust.
	(dw2_symtab_iter_next): Adjust.
	(dw2_print_stats): Adjust.
	(dw2_expand_all_symtabs): Adjust.
	(dw2_expand_symtabs_with_fullname): Adjust.
	(dw2_expand_marked_cus): Adjust.
	(dw_expand_symtabs_matching_file_matcher): Adjust.
	(dw2_map_symbol_filenames): Adjust.
	(dw2_debug_names_iterator::next): Adjust.
	(dwarf2_initialize_objfile): Adjust.
	(set_partial_user): Adjust.
	(dwarf2_build_psymtabs_hard): Adjust.