analyzer: extract bits from integer constants [PR105783]

analyzer: extract bits from integer constants [PR105783]

Fix a false positive from -Wanalyzer-null-dereference due to -fanalyzer
failing to grok the value of a particular boolean field initialized to a
constant.

gcc/analyzer/ChangeLog:
	PR analyzer/105783
	* region-model.cc (selftest::get_bit): New function.
	(selftest::test_bits_within_svalue_folding): New.
	(selfftest::analyzer_region_model_cc_tests): Call it.
	* svalue.cc (constant_svalue::maybe_fold_bits_within): Handle the
	case of extracting a single bit.

gcc/testsuite/ChangeLog:
	PR analyzer/105783
	* gcc.dg/analyzer/pr105783.c: New test.

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