rustc: Allow an edition's feature on that edition

This commit fixes a hard error where the `#![feature(rust_2018_preview)]`
feature was forbidden to be mentioned when the `--edition 2018` flag was passed.
This instead silently accepts that feature gate despite it not being necessary.
It's intended that this will help ease the transition into the 2018 edition as
users will, for the time being, start off with the `rust_2018_preview` feature
and no longer immediately need to remove it.

Closes #50662

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