Make dwarf2_per_objfile::all_type_units an std::vector

Make dwarf2_per_objfile::all_type_units an std::vector

Make all_type_units an std::vector, remove
n_type_units/n_allocated_type_units and some manual memory management.

gdb/ChangeLog:

	* dwarf2read.h (struct dwarf2_per_objfile) <n_type_units>:
	Remove.
	<n_allocated_type_units>: Remove.
	<all_type_units>: Change to std::vector.
	* dwarf2read.c (dwarf2_per_objfile::~dwarf2_per_objfile): Adjust
	to std::vector change.
	(dwarf2_per_objfile::get_cutu): Likewise.
	(dwarf2_per_objfile::get_tu): Likewise.
	(create_signatured_type_table_from_index): Likewise.
	(create_signatured_type_table_from_debug_names): Likewise.
	(dw2_symtab_iter_next): Likewise.
	(dw2_print_stats): Likewise.
	(dw2_expand_all_symtabs): Likewise.
	(dw2_expand_marked_cus): Likewise.
	(dw2_debug_names_iterator::next): Likewise.
	(dwarf2_initialize_objfile): Likewise.
	(add_signatured_type_cu_to_table): Likewise.
	(create_all_type_units): Likewise.
	(add_type_unit): Likewise.
	(struct tu_abbrev_offset): Add constructor.
	(build_type_psymtabs_1): Adjust to std::vector change.
	(print_tu_stats): Likewise.
	* dwarf-index-write.c (check_dwarf64_offsets): Likewise.
	(write_debug_names): Likewise.