Fix collapsible_match suggestion causing non-exhaustive pattern error

[JasmineLCY]

When collapsing a bare if inside a match arm into a match guard, the else arm's pattern must be a true wildcard for exhaustiveness. If the else arm is None (which arm_is_wild_like considers wild-like but isn't a true wildcard for exhaustiveness), the suggestion now replaces it with _.

Drafted with AI assistance, reviewed and tested by me.

Fixes #16860

changelog: Fix [collapsible_match] suggestion causing non-exhaustive patterns error when else arm is not a wildcard

[rustbot]

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[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • 2b1935a

[github-actions]

Lintcheck changes for de932e4

Lint	Added	Removed	Changed
clippy::collapsible_match	0	0	3

This comment will be updated if you push new changes

[samueltardieu]

Please:

• claim the issue
• don't reference issue number in commit message
• look at the lintcheck changes, one looks clearly incorrect (wildcard matching everything while other clauses follow), and include a test to ensure this is now fixed
• ensure CI is green

[rustbot]

☔ The latest upstream changes (possibly #16878) made this pull request unmergeable. Please resolve the merge conflicts.

[JasmineLCY]

Sorry for the slow response — I missed this.

I'll address each point:

1. Claiming the issue now
2. Will remove the issue number from the commit message
3. Will look into the lintcheck wildcard change and add a test
4. Will ensure CI is green

Working on it.