extract `bool_comparison` to a separate file (#15499)

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 https://github.com/rust-lang/rust-clippy/pull/15498,
but only for easier-rebase purposes.

changelog: none

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