Assumes capacity for body has already been added. Needed capacity taking into
account fixups can be found with countBodyLenAfterFixups.
fn appendBodyWithFixups(astgen: *AstGen, body: []const Zir.Inst.Index) void
fn appendBodyWithFixups(astgen: *AstGen, body: []const Zir.Inst.Index) void {
return appendBodyWithFixupsArrayList(astgen, &astgen.extra, body);
}