c++: cp_tree_equal cleanups

c++: cp_tree_equal cleanups

A couple of small fixes.  I noticed bind_template_template_parms was
not marking the parm a template parm (this broke some module
handling).  Debugging CALL_EXPR comparisons led me to refactor
cp_tree_equal's CALL_EXPR code (and my recent fix to debug printing of
same).  Finally TREE_VECS are best compared by comp_template_args.  I
recall that last piece being a left over from fixes during gcc-10.
I've been using it on the modules branch since then.

	gcc/cp/
	* tree.c (bind_template_template_parm): Mark the parm as a
	template parm.
	(cp_tree_equal): Refactor CALL_EXPR.  Use comp_template_args for
	TREE_VECs.