analyzer: validate region subclasses

analyzer: validate region subclasses

This patch converts region::validate to a vfunc, implementing
additional checking per subclass: verifying that various
region_id fields within map_region, array_region, stack_region and
root_region are valid, rather than just those within the base class.

Doing so caught bugs earlier in follow-up work I have on
canonicalization and purging of region_model.

gcc/analyzer/ChangeLog:
	* region-model.cc (region::validate): Convert model param from ptr
	to reference.  Update comment to reflect that it's now a vfunc.
	(map_region::validate): New vfunc implementation.
	(array_region::validate): New vfunc implementation.
	(stack_region::validate): New vfunc implementation.
	(root_region::validate): New vfunc implementation.
	(region_model::validate): Pass a reference rather than a pointer
	to the region::validate vfunc.
	* region-model.h (region::validate): Make virtual.  Convert model
	param from ptr to reference.
	(map_region::validate): New vfunc decl.
	(array_region::validate): New vfunc decl.
	(stack_region::validate): New vfunc decl.
	(root_region::validate): New vfunc decl.