c23: tag compatibility rules for struct and unions

c23: tag compatibility rules for struct and unions

Implement redeclaration and compatibility rules for
structures and unions in C23.

gcc/c/:
	* c-decl.cc (previous_tag): New function.
	(parser_xref_tag): Find earlier definition.
	(get_parm_info): Turn off warning for C23.
	(start_struct): Allow redefinitons.
	(finish_struct): Diagnose conflicts.
	* c-tree.h (comptypes_same_p): Add prototype.
	* c-typeck.cc (comptypes_same_p): New function.
	(comptypes_internal): Activate comparison of tagged types.
	(convert_for_assignment): Ignore qualifiers.
	(digest_init): Add error.
	(initialized_elementwise_p): Allow compatible types.

gcc/testsuite/:
	* gcc.dg/c23-enum-7.c: Remove warning.
	* gcc.dg/c23-tag-1.c: New test.
	* gcc.dg/c23-tag-2.c: New deactivated test.
	* gcc.dg/c23-tag-3.c: New test.
	* gcc.dg/c23-tag-4.c: New test.
	* gcc.dg/c23-tag-5.c: New deactivated test.
	* gcc.dg/c23-tag-6.c: New test.
	* gcc.dg/c23-tag-7.c: New test.
	* gcc.dg/c23-tag-8.c: New test.
	* gcc.dg/gnu23-tag-1.c: New test.
	* gcc.dg/gnu23-tag-2.c: New test.
	* gcc.dg/gnu23-tag-3.c: New test.
	* gcc.dg/gnu23-tag-4.c: New test.
	* gcc.dg/pr112488-2.c: Remove warning.