Allocate dwarf2_per_cu_data with 'new'

Allocate dwarf2_per_cu_data with 'new'

In a patch series I am working on, I'd like to have a non-POD member
in dwarf2_per_cu_data.  This currently can't be done because
dwarf2_per_cu_data is allocated on an obstack and initialized with
memset.

This patch changes the DWARF reader to allocate objects of this type
with 'new'.  The various "subclasses" of this type (signatured_type in
particular) are now changed to derive from dwarf2_per_cu_data, and
also use 'new' for allocation.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-04-30  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_per_bfd) <allocate_per_cu,
	allocate_signatured_type>: Change return type.
	<all_comp_units, all_type_units>: Hold unique pointers.
	(struct dwarf2_per_cu_data): Add constructor and initializers.
	(struct signatured_type): Derive from dwarf2_per_cu_data.
	* dwarf2/read.c (type_unit_group): Derive from
	dwarf2_per_cu_data.
	(dwarf2_per_bfd::get_cutu, dwarf2_per_bfd::get_cu)
	(dwarf2_per_bfd::get_tu)
	(dwarf2_per_bfd::allocate_signatured_type)
	(dwarf2_per_bfd::allocate_signatured_type)
	(create_cu_from_index_list, create_cus_from_index_list)
	(create_signatured_type_table_from_index)
	(create_signatured_type_table_from_debug_names)
	(create_addrmap_from_aranges)
	(dwarf2_base_index_functions::find_last_source_symtab)
	(dw_expand_symtabs_matching_file_matcher)
	(dwarf2_gdb_index::expand_symtabs_matching)
	(dwarf2_base_index_functions::map_symbol_filenames)
	(create_cus_from_debug_names_list)
	(dw2_debug_names_iterator::next)
	(dwarf2_debug_names_index::expand_symtabs_matching)
	(create_debug_type_hash_table, add_type_unit)
	(fill_in_sig_entry_from_dwo_entry, lookup_dwo_signatured_type):
	Update.
	(allocate_type_unit_groups_table): Use delete.
	(create_type_unit_group): Change return type.  Use new.
	(get_type_unit_group, build_type_psymtabs_1)
	(build_type_psymtab_dependencies)
	(process_skeletonless_type_unit, set_partial_user)
	(dwarf2_build_psymtabs_hard, read_comp_units_from_section)
	(create_cus_hash_table, queue_and_load_dwo_tu, follow_die_sig_1)
	(read_signatured_type): Update.
	(dwarf2_find_containing_comp_unit): Change type of
	'all_comp_units'.
	(run_test): Update.
	(dwarf2_per_bfd::allocate_per_cu)
	(dwarf2_per_bfd::allocate_signatured_type): Change return type.
	Use new.
	(add_signatured_type_cu_to_table): Update.
	* dwarf2/index-write.c (write_one_signatured_type)
	(check_dwarf64_offsets, psyms_seen_size, write_gdbindex)
	(write_debug_names): Update.