Fixes to exploded_path::feasible_p exposed a pre-existing bug
with pointer NULL-ness for pointers to symbolic_region.

symbolic_region has an "m_possibly_null" flag which if set means
that a region_svalue pointing to that region is treated as possibly
NULL.  Adding a constraint of "!= NULL" on an edge records that
the pointer is non-NULL, but doesn't affect other pointers (e.g.
if the first if a void *, but the other pointers are cast to other
pointer types).  This showed up in the tests
gcc.dg/analyzer/data-model-5b.c and -5c.c, which malloc a buffer
and test for NULL, but then cast that to a struct * and later test
that struct *: a path for the first test being non-NULL and the
second being NULL was erroneously found to be feasible.

This patch clears the m_possibly_null flag when a "!= NULL" constraint
is added, fixing that erroneous path (but not yet fixing the false
positive in the above tests, which seems to go on to hit a different
issue).  It also adds the field to dumps.

gcc/analyzer/ChangeLog:
	* program-state.cc (selftest::test_program_state_dumping): Update
	expected dump to include symbolic_region's possibly_null field.
	* region-model.cc (symbolic_region::print_fields): New vfunc
	implementation.
	(region_model::add_constraint): Clear m_possibly_null from
	symbolic_regions now known to be non-NULL.
	(selftest::test_malloc_constraints): New selftest.
	(selftest::analyzer_region_model_cc_tests): Call it.
	* region-model.h (region::dyn_cast_symbolic_region): Add non-const
	overload.
	(symbolic_region::dyn_cast_symbolic_region): Implement it.
	(symbolic_region::print_fields): New vfunc override decl.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/data-model-5b.c: Add xfail for new false
	positive leak.
	* gcc.dg/analyzer/data-model-5c.c: Likewise.
	* gcc.dg/analyzer/malloc-5.c: New test.