Rollup merge of #115867 - Zalathar:debug, r=oli-obk

coverage: Simplify internal representation of debug types

Most of these debug helper types store each of their fields as `Option<T>`, and then set them to `Some` when the relevant debug checks are enabled. This makes the struct fields awkward to read and results in some contortions when accessing the field values.

This PR addresses those problems by changing each of the helper types to have a single `state: Option<FooState>` field. Each individual method can then obtain the state up-front (or return early if it is absent), allowing the rest of the code to just access the state's contents directly.

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There are some more improvements I'd like to make to the debug code, but for this PR I'm focusing on a straightforward mechanical change that should be fairly easy to review.

(I did thrown in a few trivial changes to imports and docs, along with one switch from `FxHashMap` to `FxHashSet`.)

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Most of the changed lines are just indentation churn, so ignoring whitespace is recommended.