uiltins: Add support for clang compatible __builtin_{add,sub}c{,l,ll} [PR79173]

uiltins: Add support for clang compatible __builtin_{add,sub}c{,l,ll} [PR79173]

While the design of these builtins in clang is questionable,
rather than being say
unsigned __builtin_addc (unsigned, unsigned, bool, bool *)
so that it is clear they add two [0, 0xffffffff] range numbers
plus one [0, 1] range carry in and give [0, 0xffffffff] range
return plus [0, 1] range carry out, they actually instead
add 3 [0, 0xffffffff] values together but the carry out
isn't then the expected [0, 2] value because
0xffffffffULL + 0xffffffff + 0xffffffff is 0x2fffffffd,
but just [0, 1] whether there was any overflow at all.

It is something used in the wild and shorter to write than the
corresponding
 #define __builtin_addc(a,b,carry_in,carry_out) \
  ({ unsigned _s; \
     unsigned _c1 = __builtin_uadd_overflow (a, b, &_s); \
     unsigned _c2 = __builtin_uadd_overflow (_s, carry_in, &_s); \
     *(carry_out) = (_c1 | _c2); \
     _s; })
and so a canned builtin for something people could often use.
It isn't that hard to maintain on the GCC side, as we just lower
it to two .ADD_OVERFLOW calls early, and the already committed
pottern recognization code can then make .UADDC/.USUBC calls out of
that if the carry in is in [0, 1] range and the corresponding
optab is supported by the target.

2023-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/79173
	* builtin-types.def (BT_FN_UINT_UINT_UINT_UINT_UINTPTR,
	BT_FN_ULONG_ULONG_ULONG_ULONG_ULONGPTR,
	BT_FN_ULONGLONG_ULONGLONG_ULONGLONG_ULONGLONG_ULONGLONGPTR): New
	types.
	* builtins.def (BUILT_IN_ADDC, BUILT_IN_ADDCL, BUILT_IN_ADDCLL,
	BUILT_IN_SUBC, BUILT_IN_SUBCL, BUILT_IN_SUBCLL): New builtins.
	* builtins.cc (fold_builtin_addc_subc): New function.
	(fold_builtin_varargs): Handle BUILT_IN_{ADD,SUB}C{,L,LL}.
	* doc/extend.texi (__builtin_addc, __builtin_subc): Document.

	* gcc.target/i386/pr79173-11.c: New test.
	* gcc.dg/builtin-addc-1.c: New test.