Rollup merge of #95008 - c410-f3r:let-chains-paren, r=wesleywiser

Rollup merge of #95008 - c410-f3r:let-chains-paren, r=wesleywiser

[`let_chains`] Forbid `let` inside parentheses

Parenthesizes are mostly a no-op in let chains, in other words, they are mostly ignored.

```rust
let opt = Some(Some(1i32));

if (let Some(a) = opt && (let Some(b) = a)) && b == 1 {
    println!("`b` is declared inside but used outside");
}
```

As seen above, such behavior can lead to confusion.

A proper fix or nested encapsulation would probably require research, time and a modified MIR graph so in this PR I simply denied any `let` inside parentheses. Non-let stuff are still allowed.

```rust
fn main() {
    let fun = || true;

    if let true = (true && fun()) && (true) {
        println!("Allowed");
    }
}
```

It is worth noting that `let ...`  is not an expression and the RFC did not mention this specific situation.

cc `@matthewjasper`