c++: Adjust synthetic template parm creation

We intend to mark synthetic template parameters (coming from use of auto
parms), as DECL_VIRTUAL_P.  The API of process_template_parm is
awkwardly confusing, and we were marking the previous template parm
(unless this was the first parm).  process_template_parm returns the list
of parms, when most (all?) users really want the newly-added final node.
That's a bigger change, so let's not do it right now.  With this, we
correctly mark such synthetic parms DECL_VIRTUAL_P.

	gcc/cp/
	* parser.cc (synthesize_implicit_template_parm): Fix thinko about
	mark the new parm DECL_VIRTUAL_P.  Avoid unneccessary tree_last call.