Collapse if and if let chains in Clippy itself

[samueltardieu]

This adds a collapse_let_chains config option to the collapsible_if lint. This lint is then applies to Clippy itself. This reduces the level of indentation in Clippy source code in many locations.

The first two commits are from PR #14231, the third one from PR #14233. They must be merged first (or as part of this PR).

If reviewed on GitHub, the last two commits of this PR are best looked at side by side, with whitespace differences turned off.

changelog: none

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[samueltardieu]

Way too many hits from lintcheck, will double check.

Edit: lintcheck from the CI seems to use clippy.toml from the project, so it sets the collapse-let-chains option. I'll have to find a way to disable this during the lintcheck lint.
Edit: solved through #14233

[rustbot]

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[samueltardieu]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

Those will disappear after the PR it depends on are merged – I'll push with a linear history then, which will contain the same content.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust-clippy.git master
$ git push --force-with-lease

The following commits are merge commits (since this message was last posted):

• 36c415b

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust-clippy.git master
$ git push --force-with-lease

The following commits are merge commits (since this message was last posted):

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[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust-clippy.git master
$ git push --force-with-lease

The following commits are merge commits (since this message was last posted):

• ab5d3a3

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust-clippy.git master
$ git push --force-with-lease

The following commits are merge commits (since this message was last posted):

• 96ea1d6

[samueltardieu]

I'll put this on hold until let chains are stabilized.
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