C23: fix aliasing for structures/unions with incomplete types

C23: fix aliasing for structures/unions with incomplete types

When incomplete structure/union types are completed later, compatibility
of struct types that contain pointers to such types changes.  When forming
equivalence classes for TYPE_CANONICAL, we therefor need to be conservative
and treat all structs with the same tag which are pointer targets as
equivalent for purposed of determining equivalency of structure/union
types which contain such types as member. This avoids having to update
TYPE_CANONICAL of such structure/unions recursively. The pointer types
themselves are updated in c_update_type_canonical.

gcc/c/
	* c-typeck.cc (comptypes_internal): Add flag to track
	whether a struct is the target of a pointer.
	(tagged_types_tu_compatible): When forming equivalence
	classes, treat nested pointed-to structs as equivalent.

gcc/testsuite/
	* gcc.dg/c23-tag-incomplete-alias-1.c: New test.