Suggest `if let` when a failed `if` block could be parsed as one

[estebank]

When writing if Some(x) = foo {}, attempt to parse as if let Some(x) = foo {} for better diagnostic output and if successful provide a suggestion to add the let.

[cramertj]

Is if let the right suggestion here? Often the code would be fixed by just adding an extra =. This code is perfectly valid, for example:

fn main() {
    let x = 0;
    let foo = Some(0);
    if Some(x) == foo {}
}

[estebank]

I think it would if x wasn't in the scope. We can't know that during parsing, but the errors for

fn main() {
    let foo = Some(0);
    if Some(x) = foo {}
}

are

error[E0425]: cannot find value `x` in this scope
 --> src/main.rs:3:13
  |
3 |     if Some(x) = foo {}
  |             ^ not found in this scope

error[E0308]: mismatched types
 --> src/main.rs:3:8
  |
3 |     if Some(x) = foo {}
  |        ^^^^^^^^^^^^^
  |        |
  |        help: did you mean to compare equality?: `Some(x) == foo`
  |        expected bool, found ()
  |
  = note: expected type `bool`
             found type `()`

Which tells me we could catch not found in this scope errors that occur on if conditions and try them as if let. If it succeeds to parse the rest of the block, then emit the original error with an extra suggestion.

For the case where let x = 0 is in the code, the output is only the mismatched types error.

[estebank]

cc @Centril, given the recent it let desugaring work.

[Centril]

@estebank Added to the tracking issue for list of potential issues to work on; tho since it is preexisting, not as a blocker but a nice-to-have.