Given an index into string_bytes returns the null-terminated string found there.
pub fn nullTerminatedString(code: Zir, index: NullTerminatedString) [:0]const u8
pub fn nullTerminatedString(code: Zir, index: NullTerminatedString) [:0]const u8 {
const slice = code.string_bytes[@backingInt(index)..];
return slice[0..std.mem.findScalar(u8, slice, 0).? :0];
}