analyzer: implement bit_range_region

analyzer: implement bit_range_region

GCC 12 has gained -Wanalyzer-use-of-uninitialized-value, and I'm
seeing various false positives from it due to region_model::get_lvalue
not properly handling BIT_FIELD_REF, and falling back to
using an UNKNOWN_REGION for them.

This patch fixes these false positives by implementing a new
bit_range_region region subclass for handling BIT_FIELD_REF.

gcc/analyzer/ChangeLog:
	* analyzer.h (class bit_range_region): New forward decl.
	* region-model-manager.cc (region_model_manager::get_bit_range):
	New.
	(region_model_manager::log_stats): Handle m_bit_range_regions.
	* region-model.cc (region_model::get_lvalue_1): Handle
	BIT_FIELD_REF.
	* region-model.h (region_model_manager::get_bit_range): New decl.
	(region_model_manager::m_bit_range_regions): New field.
	* region.cc (region::get_base_region): Handle RK_BIT_RANGE.
	(region::base_region_p): Likewise.
	(region::calc_offset): Likewise.
	(bit_range_region::dump_to_pp): New.
	(bit_range_region::get_byte_size): New.
	(bit_range_region::get_bit_size): New.
	(bit_range_region::get_byte_size_sval): New.
	(bit_range_region::get_relative_concrete_offset): New.
	* region.h (enum region_kind): Add RK_BIT_RANGE.
	(region::dyn_cast_bit_range_region): New vfunc.
	(class bit_range_region): New.
	(is_a_helper <const bit_range_region *>::test): New.
	(default_hash_traits<bit_range_region::key_t>): New.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/torture/uninit-bit-field-ref.c: New test.

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