Suggest using cfg_if crate for chains of cfg(A) and cfg(not(A))

[clarfonthey]

This crate has been extremely useful for me and it would be helpful for clippy to notice these and suggest using the crate instead.

[briansmith]

Unfortunately cfg_if is problematic in some cases. I had to remove all use s of cfg_if out of a project recently because of rust-lang/rustfmt#3253. I actually am planning to have my CI/CD prevent use of cfg_if.