Fix a bug where ::warning() was called with a va_list argument instead of an actual vararg list. Also cleaned up the format for warnings. (Previously some would start with "warning - warning - Warning:" which was a bit redundant)

module bug1;
struct Foo { Foo opSub(ref Foo b) { return Foo(); } }
struct Bar { Foo whee; }
void test(ref Bar moo) { Foo nngh; auto plonk = nngh - moo.whee; }
void main() { Bar bar; test(bar); }