Auto merge of #119473 - Urgau:check-cfg-explicit-none, r=petrochenkov

Add explicit `none()` value variant in check-cfg

This PR adds an explicit none value variant in check-cfg values: `values(none())`.

Currently the only way to define the none variant is with an empty `values()` which means that if someone has a cfg that takes none and strings they need to use two invocations: `--check-cfg=cfg(foo) --check-cfg=cfg(foo, values("bar"))`.
Which would now be `--check-cfg=cfg(foo, values(none(),"bar"))`, this is simpler and easier to understand.

`--check-cfg=cfg(foo)`, `--check-cfg=cfg(foo, values())` and `--check-cfg=cfg(foo, values(none()))` would be equivalent.

*Another motivation for doing this is to make empty `values()` actually means no-values, but this is orthogonal to this PR and adding `none()` is sufficient in it-self.*

`@rustbot` label +F-check-cfg
r? `@petrochenkov`

Author: bors