ubsan: _BitInt -fsanitize=undefined support [PR102989]

ubsan: _BitInt -fsanitize=undefined support [PR102989]

The following patch introduces some -fsanitize=undefined support for _BitInt,
but some of the diagnostics is limited by lack of proper support in the
library.
I've filed https://github.com/llvm/llvm-project/issues/64100 to request
proper support, for now some of the diagnostics might have less or more
confusing or inaccurate wording but UB should still be diagnosed when it
happens.

2023-09-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/102989
gcc/
	* internal-fn.cc (expand_ubsan_result_store): Add LHS, MODE and
	DO_ERROR arguments.  For non-mode precision BITINT_TYPE results
	check if all padding bits up to mode precision are zeros or sign
	bit copies and if not, jump to DO_ERROR.
	(expand_addsub_overflow, expand_neg_overflow, expand_mul_overflow):
	Adjust expand_ubsan_result_store callers.
	* ubsan.cc: Include target.h and langhooks.h.
	(ubsan_encode_value): Pass BITINT_TYPE values which fit into pointer
	size converted to pointer sized integer, pass BITINT_TYPE values
	which fit into TImode (if supported) or DImode as those integer types
	or otherwise for now punt (pass 0).
	(ubsan_type_descriptor): Handle BITINT_TYPE.  For pstyle of
	UBSAN_PRINT_FORCE_INT use TK_Integer (0x0000) mode with a
	TImode/DImode precision rather than TK_Unknown used otherwise for
	large/huge BITINT_TYPEs.
	(instrument_si_overflow): Instrument BITINT_TYPE operations even when
	they don't have mode precision.
	* ubsan.h (enum ubsan_print_style): New enumerator.
gcc/c-family/
	* c-ubsan.cc (ubsan_instrument_shift): Use UBSAN_PRINT_FORCE_INT
	for type0 type descriptor.