c++: losslessly encode TYPENAME_TYPE tag

c++: losslessly encode TYPENAME_TYPE tag

We currently have 7 distinct tag_types, but our TYPENAME_TYPE
representation effectively only differentiates between 4 of them at the
expense of struct_type (which gets conflated with class_type),
none_type and scope_type (which get conflated with typename_type).

struct_type / class_type conflation is mostly harmless, but it
probably affects -Wmismatched-tags.  none_type / typename_type too
should be harmless.  But scope_type / typename_type conflation is
problematic because scope_type indicates a type-only lookup unlike
typename_type.

This patch makes our representation of TYPENAME_TYPE encode the numeric
tag type value losslessly using the first three adjacent tree flag bits.
We already use three flag bits for the tag encoding (but inefficiently)
so no additional space is needed.

gcc/cp/ChangeLog:

	* cp-tree.h (TYPENAME_TYPE_TAG_BIT_0): New.
	(TYPENAME_TYPE_TAG_BIT_1): New.
	(TYPENAME_TYPE_TAG_BIT_2): New.
	(TYPENAME_IS_ENUM_P): Use get_typename_tag.
	(TYPENAME_IS_CLASS_P): Rename to ...
	(TYPENAME_IS_CLASS_OR_STRUCT_P): ... this, and use
	get_typename_tag.
	(TYPENAME_IS_UNION_P): Use get_typename_tag.
	(TYPENAME_IS_RESOLVING_P): Use TREE_LANG_FLAG_3
	instead of _2.
	(get_typename_tag): New.
	(set_typename_tag): New.
	(tag_name): Declare.
	* decl.cc (typename_info): Replace bool fields with a single
	tag_types field.
	(typename_hasher::equal): Adjust.
	(build_typename_type): Adjust.
	(tag_name): Handle none_type and scope_type.
	* error.cc (dump_type) <case TYPENAME_TYPE>: Use tag_name.
	* module.cc (trees_out::type_node) <case TYPENAME_TYPE>: Use
	get_typename_tag.
	* pt.cc (tsubst) <case TYPENAME_TYPE>: Likewise.

Reviewed-by: Jason Merrill <jason@redhat.com>