This can only be for short-lived references; the memory becomes invalidated when another string is added.
fn nullTerminatedString(astgen: AstGen, index: Zir.NullTerminatedString) [*:0]const u8
fn nullTerminatedString(astgen: AstGen, index: Zir.NullTerminatedString) [*:0]const u8 {
return @ptrCast(astgen.string_bytes.items[@backingInt(index)..]);
}