Auto merge of #45452 - estebank:colon-typo, r=nikomatsakis

Auto merge of #45452 - estebank:colon-typo, r=nikomatsakis

Detect `=` -> `:` typo in let bindings

When encountering a let binding type error, attempt to parse as
initializer instead. If successful, it is likely just a typo:

```rust
fn main() {
    let x: Vec::with_capacity(10);
}
```

```
error: expected type, found `10`
 --> file.rs:3:31
  |
3 |     let x: Vec::with_capacity(10, 20);
  |         --                    ^^
  |         ||
  |         |help: did you mean assign here?: `=`
  |         while parsing the type for `x`
```

Fix #43703.