analyzer: fix false positives with *UNKNOWN_PTR [PR98918]

analyzer: fix false positives with *UNKNOWN_PTR [PR98918]

PR analyzer/98918 reports various false positives and state explosions
on correct code that frees nodes and other pointers in a singly-linked
list.

The issue is that state-merger in the loop leads to UNKNOWN_VALUEs,
and these are then erroneously used to form compound symbolic values
and regions, such as;
  INIT_VAL((*UNKNOWN(struct marker *)).ref)
and:
  (*INIT_VAL((*UNKNOWN(struct marker * *))))
The malloc state machine then treats these symbolic values as
identifying specific pointers, and thus e.g. erroneously reports a
double-free when
  INIT_VAL((*UNKNOWN(struct marker *)).ref)
is freed twice (on subsequent iterations of the loop).

Similarly, the increasingly complex compound symbolic values have
sm-state which prevents state merging, and eventually lead to the
analysis hitting safety limits and stopping.

This patch makes various compound values involving UNKNOWN be
themselves UNKNOWN, resolving both the false positives and the state
explosions.

gcc/analyzer/ChangeLog:
	PR analyzer/98918
	* region-model-manager.cc
	(region_model_manager::get_or_create_initial_value):
	Fold the initial value of *UNKNOWN_PTR to an UNKNOWN value.
	(region_model_manager::get_field_region): Fold the value
	of UNKNOWN_PTR->FIELD to *UNKNOWN_PTR_OF_&FIELD_TYPE.

gcc/testsuite/ChangeLog:
	PR analyzer/98918
	* gcc.dg/analyzer/pr98918.c: New test.