* call.c (magic_varargs_p): Return 3 for __builtin_*_overflow_p.
	(build_over_call): For magic == 3, do no conversion only on 3rd
	argument.

	* c-c++-common/torture/builtin-arith-overflow-p-19.c: Run for C++ too.
	* g++.dg/ext/builtin-arith-overflow-2.C: New test.

From-SVN: r237755