analyzer: fix ICE on initializers for unsized array fields [PR96777]

analyzer: fix ICE on initializers for unsized array fields [PR96777]

gcc/analyzer/ChangeLog:
	PR analyzer/96777
	* region-model.h (class compound_svalue): Document that all keys
	must be concrete.
	(compound_svalue::compound_svalue): Move definition to svalue.cc.
	* store.cc (binding_map::apply_ctor_to_region): Handle
	initializers for trailing arrays with incomplete size.
	* svalue.cc (compound_svalue::compound_svalue): Move definition
	here from region-model.h.  Add assertion that all keys are
	concrete.

gcc/testsuite/ChangeLog:
	PR analyzer/96777
	* gcc.dg/analyzer/pr96777.c: New test.