Additional small changes to support opaque modes

After building some larger codes using opaque types and some c++ codes
using opaque types it became clear I needed to go through and look for
places where opaque types and modes needed to be handled. A whole pile
of one-liners.

gcc/
	* typeclass.h: Add opaque_type_class.
	* builtins.c (type_to_class): Identify opaque type class.
	* dwarf2out.c (is_base_type): Handle opaque types.
	(gen_type_die_with_usage): Handle opaque types.
	* expr.c (count_type_elements): Opaque types should
	never have initializers.
	* ipa-devirt.c (odr_types_equivalent_p): No type-specific handling
	for opaque types is needed as it eventually checks the underlying
	mode which is what is important.
	* tree-streamer.c (record_common_node): Handle opaque types.
	* tree.c (type_contains_placeholder_1): Handle opaque types.
	(type_cache_hasher::equal): No additional comparison needed for
	opaque types.
gcc/c-family
	* c-pretty-print.c (c_pretty_printer::simple_type_specifier):
	Treat opaque types like other types.
	(c_pretty_printer::direct_abstract_declarator): Opaque types are
	supported types.
gcc/c
	* c-aux-info.c (gen_type): Support opaque types.
gcc/cp
	* error.c (dump_type): Handle opaque types.
	(dump_type_prefix): Handle opaque types.
	(dump_type_suffix): Handle opaque types.
	(dump_expr): Handle opaque types.
	* pt.c (tsubst): Allow opaque types in templates.
	(unify): Allow opaque types in templates.
	* typeck.c (structural_comptypes): Handle comparison
	of opaque types.