analyzer: add class region_to_value_map

Add a class for associating symbolic values with regions, for use
initially for recording the sizes of dynamically-allocated regions,
though this also could potentially be used for e.g. tracking strlen()
values.

gcc/analyzer/ChangeLog:
	* region-model.cc (region_to_value_map::operator=): New.
	(region_to_value_map::operator==): New.
	(region_to_value_map::dump_to_pp): New.
	(region_to_value_map::dump): New.
	(region_to_value_map::can_merge_with_p): New.
	* region-model.h (class region_to_value_map): New class.

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