c++: simplify TEMPLATE_TYPE_PARM level lowering

1. Don't bother recursing when level lowering a cv-qualified type
   template parameter.
2. Get rid of the recursive loop breaker when level lowering a
   constrained auto, and enable the TEMPLATE_PARM_DESCENDANTS cache in
   this case too.  This should be safe to do so now that we no longer
   substitute constraints on an auto.

gcc/cp/ChangeLog:

	* pt.cc (tsubst) <case TEMPLATE_TYPE_PARM>: Don't recurse when
	level lowering a cv-qualified type template parameter.  Remove
	recursive loop breaker in the level lowering case for constrained
	autos.  Use the TEMPLATE_PARM_DESCENDANTS cache in this case as
	well.