Specialize the "match arms have incompatible types" error for if let

[jonas-schievink]

Closes #29314

The code from #29314:

fn main() {
    if let Some(b) = None {
        ()
    } else {
        1
    };
}

now prints this:

test.rs:2:5: 6:6 error: `if let` arms have incompatible types: expected `()`, found `_` (expected (), found integral variable) [E0308]
test.rs:2     if let Some(b) = None {
test.rs:3         ()
test.rs:4     } else {
test.rs:5         1
test.rs:6     };
test.rs:2:5: 6:6 help: run `rustc --explain E0308` to see a detailed explanation
test.rs:4:12: 6:6 note: `if let` arm with an incompatible type
test.rs:4     } else {
test.rs:5         1
test.rs:6     };
error: aborting due to previous error

[rust-highfive]

r? @jroesch

(rust_highfive has picked a reviewer for you, use r? to override)

[arielb1]

@bors r+ 2560646

[jonas-schievink]

@arielb1 Looks like bors didn't catch that

[arielb1]

@bors r+

[bors]

📌 Commit 2560646 has been approved by arielb1

[bors]

⌛ Testing commit 2560646 with merge 88fade5...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt