c++: ICE with failed __is_constructible constraint [PR100474]

c++: ICE with failed __is_constructible constraint [PR100474]

Here we're crashing when diagnosing an unsatisfied __is_constructible
constraint because diagnose_trait_expr doesn't recognize this trait
(along with a bunch of other traits).  Fix this by adding handling for
all remaining traits and removing the default case so that when adding a
new trait we'll get a warning that diagnose_trait_expr needs to handle it.

	PR c++/100474

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Handle all remaining
	traits appropriately.  Remove default case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-traits3.C: New test.