Rollup merge of #111068 - Urgau:check-cfg-improvements, r=petrochenkov

Improve check-cfg implementation

This PR makes multiple improvements into the implementation of check-cfg, it is a prerequisite to a follow-up PR that will introduce a simpler and more explicit syntax.

The 2 main area of improvements are:
 1. Internal representation of expected values:
    - now uses `FxHashSet<Option<Symbol>>` instead of `FxHashSet<Symbol>`, it made the no value expected case only possible when no values where in the `HashSet` which is now represented as `None` (same as cfg represent-it).
    - a enum with `Some` and `Any` makes it now clear if some values are expected or not, necessary for `feature` and `target_feature`.
 2. Diagnostics: Improve the diagnostics in multiple case and fix case where a missing value could have had a new name suggestion instead of the value diagnostic; and some drive by improvements

I highly recommend reviewing commit by commit.

r? `@petrochenkov`

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