c++: TI_DEFERRED_ACCESS_CHECKS and dependent decls

c++: TI_DEFERRED_ACCESS_CHECKS and dependent decls

This adds an assert to enforce_access to verify that we don't defer
access checks of dependent decls -- we should instead be rechecking the
access of such a decl after tsubst'ing into the user of the decl.

gcc/cp/ChangeLog:

	* pt.c (perform_instantiation_time_access_checks): No need to
	tsubst into decl.
	* semantics.c (enforce_access): Verify that decl is not
	dependent.