Sema: implement peer type resolution of signed and unsigned ints
This allows stage2 to build more of compiler-rt.
I also changed `-%` to `-` for comptime ints in the div and mul
implementations of compiler-rt. This is clearer code and also happens to
work around a bug in stage2.
b4d6e85a33Andrew Kelley committed on 1/2/2022, 9:11:37 PMparent6b14c58