c++: Clean up previous change [PR41437]

c++: Clean up previous change [PR41437]

The previous patch mostly avoided making any changes that had no
functional impact, such as adjusting now-outdated comments and
performing renamings.  Such changes have been consolidated to this
followup patch for easier review.

The main change here is that we now reuse struct deferred_access_check
as the element type of the vector TI_TYPEDEFS_NEEDING_ACCESS_CHECKING
(now renamed to TI_DEFERRED_ACCESS_CHECKS, since it may contain any kind
of access check).

gcc/cp/ChangeLog:

	PR c++/41437
	PR c++/47346
	* cp-tree.h (qualified_typedef_usage_s): Delete.
	(qualified_typedef_usage_t): Delete.
	(deferred_access_check): Move up in file.
	(tree_template_info::typedefs_needing_access_checking): Delete.
	(tree_template_info::deferred_access_checks): New field.
	(TI_TYPEDEFS_NEEDING_ACCESS_CHECKING): Rename to ...
	(TI_DEFERRED_ACCESS_CHECKS): ... this, and adjust accordingly.
	* pt.c (perform_typedefs_access_check): Rename to ...
	(perform_instantiation_time_access_checks): ... this, and adjust
	accordingly.  Remove unnecessary tree tests.
	(instantiate_class_template_1): Adjust accordingly.
	(instantiate_decl): Likewise.
	* semantics.c (enforce_access): Likewise.