Detect incorrect chaining of `if` and `if let` conditions and recover gracefully

[estebank]

Given

fn foo(b: bool, x: Option<u32>) {
    if b && if let Some(x) = x {}
}
fn main() {
    if true && if true { true }
}

the current output is

error: expected `{`, found `}`
 --> src/main.rs:3:1
  |
3 | }
  | ^ expected `{`
  |
note: the `if` expression is missing a block after this condition
 --> src/main.rs:2:8
  |
2 |     if b && if let Some(x) = x {}
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^

error: expected `{`, found `}`
 --> src/main.rs:6:1
  |
6 | }
  | ^ expected `{`
  |
note: the `if` expression is missing a block after this condition
 --> src/main.rs:5:8
  |
5 |     if true && if true { true }
  |        ^^^^^^^^^^^^^^^^^^^^^^^^

We should detect that what was intended was

fn foo(b: bool, x: Option<u32>) {
    if b && let Some(x) = x {}
}
fn main() {
    if true && true { true }
}

#53667 (comment)

[chenyukang]

@rustbot claim

[est31]

@rustbot label F-let_chains