C: allow aliasing of compatible types derived from enumeral types [PR115157]

C: allow aliasing of compatible types derived from enumeral types [PR115157]

Aliasing of enumeral types with the underlying integer is now allowed
by setting the aliasing set to zero.  But this does not allow aliasing
of derived types which are compatible as required by ISO C.  Instead,
initially set structural equality.  Then set TYPE_CANONICAL and update
pointers and main variants when the type is completed (as done for
structures and unions in C23).

	PR tree-optimization/115157
	PR tree-optimization/115177

gcc/c/
	* c-decl.cc (shadow_tag-warned,parse_xref_tag,start_enum,
	finish_enum): Set SET_TYPE_STRUCTURAL_EQUALITY / TYPE_CANONICAL.
	* c-objc-common.cc (get_alias_set): Remove special case.
	(get_aka_type): Add special case.

gcc/c-family/
	* c-attribs.cc (handle_hardbool_attribute): Set TYPE_CANONICAL
	for hardbools.

gcc/
	* godump.cc (go_output_typedef): Use TYPE_MAIN_VARIANT instead
	of TYPE_CANONICAL.

gcc/testsuite/
	* gcc.dg/enum-alias-1.c: New test.
	* gcc.dg/enum-alias-2.c: New test.
	* gcc.dg/enum-alias-3.c: New test.
	* gcc.dg/enum-alias-4.c: New test.