c++: make some cp_trait_kind switch statements exhaustive

c++: make some cp_trait_kind switch statements exhaustive

This replaces the unreachable default case in some cp_trait_kind
switches with an exhaustive listing of the trait codes that we don't
expect to see, so that when adding a new trait we'll get a helpful
-Wswitch warning if we forget to handle the new trait in a relevant
switch.

gcc/cp/ChangeLog:

	* semantics.cc (trait_expr_value): Make cp_trait_kind switch
	statement exhaustive.
	(finish_trait_expr): Likewise.
	(finish_trait_type): Likewise.