c++: make trait of incomplete type a permerror [PR109277]

c++: make trait of incomplete type a permerror [PR109277]

An incomplete type argument to several traits is specified to be undefined
behavior in the library; since it's a compile-time property, we diagnose
it.  But apparently some code was relying on the previous behavior of not
diagnosing.  So let's make it a permerror.

The assert in cxx_incomplete_type_diagnostic didn't like that, and I don't
see the point of having the assert, so let's just remove it.

	PR c++/109277

gcc/cp/ChangeLog:

	* semantics.cc (check_trait_type): Handle incomplete type directly.
	* typeck2.cc (cxx_incomplete_type_diagnostic): Remove assert.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible5.C: New test.