c23: aliasing of compatible tagged types

Tell the backend which types are equivalent by setting
TYPE_CANONICAL to one struct in the set of equivalent
structs.  Structs are considered equivalent by ignoring
all sizes of arrays nested in types below field level.

The following two structs are incompatible and lvalues
with these types can be assumed not to alias:

 struct foo { int a[3]; };
 struct foo { int a[4]; };

The following two structs are also incompatible, but
will get the same TYPE_CANONICAL and it is then not
exploited that lvalues with those types can not alias:

 struct bar { int (*p)[3]; };
 struct bar { int (*p)[4]; };

The reason is that both are compatible to

 struct bar { int (*p)[]; };

and therefore are in the same equivalence class.  For
the same reason all enums with the same underyling type
are in the same equivalence class.  Tests are added
for the expected aliasing behavior with optimization.

gcc/c:
	* c-decl.cc (c_struct_hasher): Hash stable for struct
	types.
	(c_struct_hasher::hash, c_struct_hasher::equal): New
	functions.
	(finish_struct): Set TYPE_CANONICAL to first struct in
	equivalence class.
	* c-objc-common.cc (c_get_alias_set): Let structs or
	unions with variable size alias anything.
	* c-tree.h (comptypes_equiv): New prototype.
	* c-typeck.cc (comptypes_equiv): New function.
	(comptypes_internal): Implement equivalence mode.
	(tagged_types_tu_compatible): Implement equivalence mode.

gcc/testsuite:
	* gcc.dg/c23-tag-2.c: Activate.
	* gcc.dg/c23-tag-5.c: Activate.
	* gcc.dg/c23-tag-alias-1.c: New test.
	* gcc.dg/c23-tag-alias-2.c: New test.
	* gcc.dg/c23-tag-alias-3.c: New test.
	* gcc.dg/c23-tag-alias-4.c: New test.
	* gcc.dg/c23-tag-alias-5.c: New test.
	* gcc.dg/gnu23-tag-alias-1.c: New test.
	* gcc.dg/gnu23-tag-alias-2.c: New test.
	* gcc.dg/gnu23-tag-alias-3.c: New test.
	* gcc.dg/gnu23-tag-alias-4.c: New test.
	* gcc.dg/gnu23-tag-alias-5.c: New test.
	* gcc.dg/gnu23-tag-alias-6.c: New test.
	* gcc.dg/gnu23-tag-alias-7.c: New test.

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