"using _ for array lengths is unstable" error should suggest the number of elements

[Kampfkarren]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=fe35b2f5f585f0303661ac61e880d1fe

const NUMBERS: [i32; _] = [
    1, 2, 3
];

The current output is:

error[[E0658]](https://doc.rust-lang.org/stable/error-index.html#E0658): using `_` for array lengths is unstable
 --> src/lib.rs:1:22
  |
1 | const NUMBERS: [i32; _] = [
  |                      ^
  |
  = note: [see issue #85077 <https://github.com/rust-lang/rust/issues/85077>](https://github.com/rust-lang/rust/issues/85077) for more information

error: in expressions, `_` can only be used on the left-hand side of an assignment
 --> src/lib.rs:1:22
  |
1 | const NUMBERS: [i32; _] = [
  |                      ^ `_` not allowed here

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground` due to 2 previous errors

Ideally, the output should suggest that this should be [i32; 3].

[WaffleLapkin]

This can use diagnostic stashing in ast lowering, so that later, in type check, when we can infer the infer constant we can provide a better diagnostic. I haven't found where in type check to do this yet :')