analyzer: widening_svalues take a function_point rather than a program_point

analyzer: widening_svalues take a function_point rather than a program_point

Enabling work towrads better call summarization.

gcc/analyzer/ChangeLog:
	* region-model-manager.cc
	(region_model_manager::get_or_create_widening_svalue): Use a
	function_point rather than a program_point.
	* region-model.cc (selftest::test_widening_constraints): Likewise.
	* region-model.h
	(region_model_manager::get_or_create_widening_svalue): Likewise.
	(model_merger::get_function_point): New.
	* svalue.cc (svalue::can_merge_p): Use a function_point rather
	than a program_point.
	(svalue::can_merge_p): Likewise.
	* svalue.h (widening_svalue::key_t): Likewise.
	(widening_svalue::widening_svalue): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>