Ensure return statement has value

[pzipper]

The following program crashes the compiler:

const Printf = func printf(&u8): i32;

const Main = func main(): i32 {
    Printf("Hello, world!");
    return;
};

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Compilation(Verifier(VerifierErrors([VerifierError { location: inst2, context: Some("return"), message: "arguments of return must match function signature" }])))', src\clif\mod.rs:328:22

Instead, a nice diagnostic should be shown.