extract bool_comparison to a separate file

[ada4a]

For some reason I don't really understand, this lint was mushed together with needless_bool(_assign), even though they seemingly didn't have a lot of overlapping logic -- for one, their lint passes are separate, and can't be merged in a meaningful way.

I first wanted to move the file under booleans/ and move the common logic into utils.rs, and then split the files, but it turns out that the entirety of common logic was the fetch_bool_expr function, which is literally just expr.as_literal()?.as_bool(). So I scratched that idea, moved bool_comparison logic to a new file, and copy-pasted the function into it.

I think this change is advantageous in two main ways:

1. no confusing intermingling of two different passes' logics
2. the file that defines clippy::bool_comparison becomes easier to find, because now there's a file with that name.

This PR depends on #15498, but only for easier-rebase purposes.

changelog: none

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[github-actions]

No changes for f0563ad

[ada4a]

I'm not^^ let's see if this will work..

@rustbot label S-waiting-on-review -A-lint -S-blocked

[samueltardieu]

@rustbot note remove Feature-freeze

[samueltardieu]

r? samueltardieu
@rustbot blocked
(by #15513)