add unreachable_cfgs lint

[folkertdev]

tracking issue: #115585

Split out from #149783. This lint is emitted on branches of a cfg_select! that are statically known to be unreachable. The lint is only emitted when the feature is enabled, so this change specifically does not need an FCP, and the lint will be stabilized alongside the feature (see #149783 (comment)).

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[rustbot]

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[Urgau]

The proposed name for this lint seems quite unfortunate to me. When I first read it I though it had something to do with the #[cfg] attribute, like detecting #[cfg] predicates that are unreachable.

I think it would be better include the word "arm" or "cfg_select" in it, to clearly disambiguate it from the other places where cfgs can appear.

Maybe unreachable_cfg_arms or unreachable_cfg_select_arms?

[folkertdev]

There is some reasoning for then name at #149783 (comment).

cc @traviscross was there any particular reason for you to leave out the select or arm/branch parts?