Replace if_chain with let chains

[Alexendoo]

Closes #9353

Let chains are now supported by rustfmt 🎉

The PR is split into two commits

1. The result of running if-to-let-chain clippy*/**/*.rs
2. The manual clean up: fixing some errors/formatting, dogfood lints, removing the if_chain internal lint

r? @flip1995

changelog: none

[bors]

☔ The latest upstream changes (presumably #11780) made this pull request unmergeable. Please resolve the merge conflicts.

[flip1995]

I will review this today. No need to rebase before that. Once I'm done reviewing, you can rebase and directly merge

[flip1995]

LGTM overall. rg if_chain in the repo still shows a few if_chain usages. But those are in tests and necessary. However, I think once let-chains are stabilized, we can also remove this one special check for if_chain in the index_refutable_slice lint implementation.

The needless_borrow_pat test implements its own if_chain macro for testing. Maybe we can reuse that in index_refutable_slice for now and with that get rid of the if_chain dep in the top level Cargo.toml.

All of this is non-blocking for this PR. Feel free to address it here or we can deal with it in a follow-up PR. r=me after rebase

[Alexendoo]

@bors r=flip1995

The if_chain implementation from index_refutable_slice didn't seem to trigger the lint so I've left the if_chain dev dep there for now

[bors]

📌 Commit 13b4bb1 has been approved by flip1995

It is now in the queue for this repository.

[bors]

⌛ Testing commit 13b4bb1 with merge 9a4dd10...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 9a4dd10 to master...