c++: add __is_*destructible builtins [PR107600]

c++: add __is_*destructible builtins [PR107600]

Typically "does this class have a trivial destructor" is the wrong question
to ask, we rather want "can I destroy this class trivially", thus the
std::is_trivially_destructible standard trait.  Let's provide a builtin for
it, and complain about asking whether a deleted destructor is trivial.

Clang and MSVC also have these traits.

	PR c++/107600

gcc/cp/ChangeLog:

	* cp-trait.def (IS_DESTRUCTIBLE, IS_NOTHROW_DESTRUCTIBLE)
	(IS_TRIVIALLY_DESTRUCTIBLE): New.
	* constraint.cc (diagnose_trait_expr): Explain them.
	* method.cc (destructible_expr): New.
	(is_xible_helper): Use it.
	* semantics.cc (finish_trait_expr): Handle new traits.
	(trait_expr_value): Likewise.  Complain about asking
	whether a deleted dtor is trivial.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_destructible1.C: New test.