analyzer: add region_model::check_region_access

analyzer: add region_model::check_region_access

I've been experimenting with various new diagnostics that
require a common place for the analyzer to check the validity
of reads or writes to memory (e.g. buffer overflow).

As preliminary work, this patch adds new
  region_model::check_region_for_{read|write} functions
which are called anywhere that the analyzer "sees" memory being
read from or written to (via region_model::get_store_value and
region_model::set_value).

This takes over the hardcoded calls to check_for_writable_region
(allowing for other kinds of checks on writes); checking reads is
currently a no-op.

gcc/analyzer/ChangeLog:
	* analyzer.h (enum access_direction): New.
	* engine.cc (exploded_node::on_longjmp): Update for new param of
	get_store_value.
	* program-state.cc (program_state::prune_for_point): Likewise.
	* region-model-impl-calls.cc (region_model::impl_call_memcpy):
	Replace call to check_for_writable_region with call to
	check_region_for_write.
	(region_model::impl_call_memset): Likewise.
	(region_model::impl_call_strcpy): Likewise.
	* region-model-reachability.cc (reachable_regions::add): Update
	for new param of get_store_value.
	* region-model.cc (region_model::get_rvalue_1): Likewise, also for
	get_rvalue_for_bits.
	(region_model::get_store_value): Add ctxt param and use it to call
	check_region_for_read.
	(region_model::get_rvalue_for_bits): Add ctxt param and use it to
	call get_store_value.
	(region_model::check_region_access): New.
	(region_model::check_region_for_write): New.
	(region_model::check_region_for_read): New.
	(region_model::set_value): Update comment.  Replace call to
	check_for_writable_region with call to check_region_for_write.
	* region-model.h (region_model::get_rvalue_for_bits): Add ctxt
	param.
	(region_model::get_store_value): Add ctxt param.
	(region_model::check_region_access): New decl.
	(region_model::check_region_for_write): New decl.
	(region_model::check_region_for_read): New decl.
	* region.cc (region_model::copy_region): Update call to
	get_store_value.
	* svalue.cc (initial_svalue::implicitly_live_p): Likewise.

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