analyzer: fix infinite recursion ICE on unions [PR96723]

analyzer: fix infinite recursion ICE on unions [PR96723]

Attempts to store sm-state into a union in C++ triggered an infinite
recursion when trying to generate a representative tree, due to
erroneously trying to use the dtor of the union as a field.

Fix it by filtering out non-FIELD_DECLs when walking TYPE_FIELDs
in region::get_subregions_for_binding.

gcc/analyzer/ChangeLog:
	PR analyzer/96723
	* region-model-manager.cc
	(region_model_manager::get_field_region): Assert that field is a
	FIELD_DECL.
	* region.cc (region::get_subregions_for_binding): In
	union-handling, filter the TYPE_FIELDS traversal to just FIELD_DECLs.

gcc/testsuite/ChangeLog:
	PR analyzer/96723
	* g++.dg/analyzer/pr96723.C: New test.