analyzer: introduce noop_region_model_context

analyzer: introduce noop_region_model_context

tentative_region_model_context and test_region_model_context are both
forced to implement numerous pure virtual vfuncs of the abstract
region_model_context.

This patch adds a noop_region_model_context which provides empty
implementations of all of region_model_context's pure virtual functions,
and subclasses the above classes from that, rather than from
region_model_context directly.

gcc/analyzer/ChangeLog:
	* region-model.h (class noop_region_model_context): New subclass
	of region_model_context.
	(class tentative_region_model_context): Inherit from
	noop_region_model_context rather than from region_model_context;
	drop redundant vfunc implementations.
	(class test_region_model_context): Likewise.