E0408 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0408.rs

E0408 needs a span_label, updating it from:

error[E0408]: variable `y` from pattern #1 is not bound in pattern #2
  --> src/test/compile-fail/E0408.rs:15:19
   |
15 |         Some(y) | None => {} //~ ERROR E0408
   |                   ^^^^

To:

error[E0408]: variable `y` from pattern #1 is not bound in pattern #2
  --> src/test/compile-fail/E0408.rs:15:19
   |
15 |         Some(y) | None => {} //~ ERROR E0408
   |                   ^^^^ pattern doesn't bind `y`

[faebser]

I'm gonna work on this, Part of #35233

[bstrie]

I'm going through and freeing up issues for #35233, @faebser are you still interested in finishing this? If not, other people may be interested in basing their work off of @faebser's current PR.

[faebser]

@bstrie: I am still working on the PR over here: #36307

[estebank]

This issue can be closed (changed in #39713):

error[E0408]: variable `y` is not bound in all patterns
 --> <anon>:5:19
  |
5 |         Some(y) | None => {}
  |              -    ^^^^ pattern doesn't bind `y`
  |              |
  |              variable not in all patterns