Collect occurrences of `{}` for mismatched braces diagnostic

[estebank]

Many editors helpfully auto close delimiters when writing them down. It is not uncommon for this to cause the following to occur:

fn main() {}
    let _ = ();
}

It'd be a good idea for the parser or lexer to keep track of these empty blocks to provide an error closer to

error: unexpected close delimiter: `}`
 --> src/main.rs:3:1
  |
1 | fn main() {}
  |           -- this block is empty, you might have not meant to close it
2 |     let _ = ();
3 | }
  | ^ unexpected close delimiter

[PramodBisht]

@estebank can I take this on, if nobody is working on this?

[estebank]

@PramodBisht go ahead!

[XiangQingW]

@PramodBisht Are you still working on this? I'm happy to take it if not.

[PramodBisht]

@XiangQingW Please allow me 2-3 days to check this issue.

[PramodBisht]

@XiangQingW Please take this forward.

[XiangQingW]

@XiangQingW Please take this forward.

Ok, I'll try to dive into.