Tweak irrefutable let else warning output

[WeiTheShinobi]

Fixes #153454

Greeting!
This PR tweak diagnostic output for irrefutable-let-else patterns and adds a check for let a = Some(b) else {...}
Thanks for the review!

help: consider using `let Some(name) = case` to match on a specific variant
   |
LL -     let name = Some(case) else {
LL +     let Some(name) = case else {
   |

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

[rustbot]

r? @davidtwco

rustbot has assigned @davidtwco.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 69 candidates
• Random selection from 13 candidates

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.