c++: Formatting fixups & some simplifications.

c++: Formatting fixups & some simplifications.

A bunch of minor reformatting, simplifications or change to checking_asserts.

	* pt.c (spec_hash_table): New typedef.
	(decl_specializations, type_specializations): Use it.
	(retrieve_specialization): Likewise.
	(register_specialization): Remove unnecessary casts.
	(push_template_decl_real): Reformat.
	(instantiate_class_template_1): Use more RAII.
	(make_argument_pack): Simplify.
	(instantiate_template_1): Use gcc_checking_assert for expensive
	asserts.
	(instantiate_decl): Likewise.
	(resolve_typename_type): Reformat comment.
	* semantics.c (struct deferred_access): Remove unnecessary GTY on
	member.
	(begin_class_definition): Fix formatting.