E0081 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0081.rs

Error E0081 needs a span_label and a span_note replaced with span_label, updating it from:

error[E0081]: discriminant value `3isize` already exists
  --> src/test/compile-fail/E0081.rs:13:5
   |
13 |     X = 3, //~ ERROR E0081
   |     ^^^^^
   |
note: conflicting discriminant here
  --> src/test/compile-fail/E0081.rs:12:5
   |
12 |     P = 3,
   |     ^^^^^

to:

error[E0081]: discriminant value `3isize` already exists
  --> src/test/compile-fail/E0081.rs:13:5
   |
12 |     P = 3,
   |     ----- first use of `3isize`
13 |     X = 3, //~ ERROR E0081
   |     ^^^^^ enum already has `3isize`

Bonus: this may be nicer with spans that focus on the value.

error[E0081]: discriminant value `3isize` already exists
  --> src/test/compile-fail/E0081.rs:13:5
   |
12 |     P = 3,
   |         - first use of `3isize`
13 |     X = 3, //~ ERROR E0081
   |         ^ enum already has `3isize`

[KiChjang]

Looks like #35353 didn't attempt the bonus. @poveda-ruiz are you going to finish this? Otherwise, I'm taking over.

Take it over, I don't know how to do it :)