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.