analyzer: fix initialization from constant pool [PR96609,PR96616]

analyzer: fix initialization from constant pool [PR96609,PR96616]

PR testsuite/96609 and PR analyzer/96616 report various testsuite
failures seen on powerpc64, aarch64, and arm in new tests added by
r11-2694-g808f4dfeb3a95f50f15e71148e5c1067f90a126d.

Some of these failures (in gcc.dg/analyzer/init.c, and on arm
in gcc.dg/analyzer/casts-1.c) relate to initializations from var_decls
in the constant pool.  I wrote the tests assuming that the gimplified
stmts would initialize the locals via a gassign of code CONSTRUCTOR,
whereas on these targets some of the initializations are gassign from
a VAR_DECL e.g.:
  c = *.LC0;
where "*.LC0" is a var_decl with DECL_IN_CONSTANT_POOL set.

For example, in test_7:
   struct coord c[2] = {{3, 4}, {5, 6}};
   __analyzer_eval (c[0].x == 3); /* { dg-warning "TRUE" } */
after the initialization, the store was simply recording:
   cluster for: c: INIT_VAL(*.LC0)
when I was expecting the cluster for c to have:
  cluster for: c
    key:   {kind: direct, start: 0, size: 32, next: 32}
    value: 'int' {(int)3}
    key:   {kind: direct, start: 32, size: 32, next: 64}
    value: 'int' {(int)4}
    key:   {kind: direct, start: 64, size: 32, next: 96}
    value: 'int' {(int)5}
    key:   {kind: direct, start: 96, size: 32, next: 128}
    value: 'int' {(int)6}
The test for c[0].x == 3 would then generate:
  cluster for: _2: (SUB(SUB(INIT_VAL(*.LC0), c[(int)0]), c[(int)0].x)==(int)3)
which is UNKNOWN, leading to the test failing.

This patch fixes the init.c and casts-1.c failures by special-casing
reads from a var_decl with DECL_IN_CONSTANT_POOL set, so that they build
a compound_svalue containing the bindings implied by the CONSTRUCTOR
node for DECL_INITIAL.

gcc/analyzer/ChangeLog:
	PR testsuite/96609
	PR analyzer/96616
	* region-model.cc (region_model::get_store_value): Call
	maybe_get_constant_value on decl_regions first.
	* region-model.h (decl_region::maybe_get_constant_value): New decl.
	* region.cc (decl_region::get_stack_depth): Likewise.
	(decl_region::maybe_get_constant_value): New.
	* store.cc (get_subregion_within_ctor): New.
	(binding_map::apply_ctor_to_region): New.
	* store.h (binding_map::apply_ctor_to_region): New decl.