`map_identity`: suggest making the variable mutable when necessary (#15268)

While working on rust-lang/rust-clippy#15229, I noticed a test case in
`map_identity` which avoids linting in the case where removing the
`.map()` would require the iterator variable to be made mutable. But
then I saw rust-lang/rust-clippy#14140, and thought I'd try to adapt its
approach, by suggesting both removing the `.map()` _and_ making the
variable mutable.

This is WIP only because I'm not sure about the very last
`diag.span_note` -- I think having a method chained immediately after
the `.map()` is the only case which requires adding `mut`, so it should
be safe to say that `method_requiring_mut` is always `Some`. I'd like to
do a lintcheck run to see if that's actually true.

@samueltardieu do you think this is a good approach? Maybe there are
more lints where we could lint + suggest making the variable mut,
instead of not linting?

changelog:`map_identity`: suggest making the variable mutable when
necessary

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