c++: clobber non-placement new

c++: clobber non-placement new

And also add the clobber for non-placement new.

For now let's limit the clobber of an array with non-constant bound to
placement new in constant evaluation, where we need it to set the active
member of a union.

And catch some additional cases of there being no actual data to clobber.

This changes the diagnostics in a couple of analyzer tests, but the new
diagnostics are also valid.

It also adds some -Wuninitialized warnings which seem like an improvement;
the lines that now warn about an uninitialized vptr are correct, since
trying to assign to a member of a virtual base reads the vptr of an object
that was never created.

gcc/cp/ChangeLog:

	* init.cc (build_new_1): Also clobber for non-placement new.
	Only loop clobber in constexpr.
	* expr.cc (wrap_with_if_consteval): New.
	* cp-tree.h (wrap_with_if_consteval): Declare.

gcc/testsuite/ChangeLog:

	* g++.dg/analyzer/new-2.C: Adjust diags.
	* g++.dg/analyzer/noexcept-new.C: Adjust diags.
	* g++.dg/warn/Warray-bounds-23.C: Add warnings.
	* g++.dg/warn/Warray-bounds-24.C: Add warnings.
	* g++.dg/cpp26/constexpr-new4a.C: New test.