c++, abi: Fix abi_tag attribute handling [PR98481]

c++, abi: Fix abi_tag attribute handling [PR98481]

In GCC10 cp_walk_subtrees has been changed to walk template arguments.
As the following testcase, that changed the mangling of some functions.
I believe the previous behavior that find_abi_tags_r doesn't recurse into
template args has been the correct one, but setting *walk_subtrees = 0
for the types and handling the types subtree walking manually in
find_abi_tags_r looks too hard, there are a lot of subtrees and details what
should and shouldn't be walked, both in tree.c (walk_type_fields there,
which is static) and in cp_walk_subtrees itself.

The following patch abuses the fact that *walk_subtrees is an int to
tell cp_walk_subtrees it shouldn't walk the template args.

Co-authored-by: Jason Merrill <jason@redhat.com>
gcc/cp/ChangeLog:

	PR c++/98481
	* class.c (find_abi_tags_r): Set *walk_subtrees to 2 instead of 1
	for types.
	(mark_abi_tags_r): Likewise.
	* decl2.c (min_vis_r): Likewise.
	* tree.c (cp_walk_subtrees): If *walk_subtrees_p is 2, look through
	typedefs.

gcc/testsuite/ChangeLog:

	PR c++/98481
	* g++.dg/abi/abi-tag24.C: New test.