Add a new set of interfaces to hide runtime members during model building

[AndriySvyryd]

Part of #19213

The PRs will get smaller after this one

[roji]

We now have two concepts of read-only-ness:

• In the interface hierarchy (read-only = does not (necessarily) support runtime annotations)
• In the model's state (read-only = model has been finalized, no longer mutatable)

If I'm understanding everything right, they're also opposites: finalization also roughly corresponds to transitioning from IReadOnlyModel to IModel; so the model becomes read-only in one sense (can no longer mutate it because it's finalized), and is kinda no longer read-only in the other (we can now put runtime annotations on it).

I guess if we found another name it would be slightly better, e.g. IBuildTimeModel instead of IReadOnlyModel.

My personal preference would be to simply call it IModelBase, IIndexBase, which also avoids the confusion of "why does mutable inherit from read-only". We would then need to rename ITypeBase and IPropertyBase to, say, IRuntimeTypeBase and IRuntimeTypeBase - that would technically be a breaking change, though only in intermediate interfaces so unlikely to have real impact.

Also OK with leaving as-is :)

[AndriySvyryd]

Personally I like IReadOnlyModel because it doesn't expose any mutation API, independently of the state of the implementing objects. This follows the IList/IReadOnlyList convention.

The model state IsReadOnly is really an internal concept, with the new interfaces callers won't run into readonly/mutable exceptions unless they cast to a different interface, so they don't need to be concerned about it.

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[ghost]

Apologies, while this PR appears ready to be merged, I've been configured to only merge when all checks have explicitly passed. The following integrations have not reported any progress on their checks and are blocking auto-merge:

1. Azure Pipelines

These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

Give feedback on this

From the bot dev team

We've tried to tune the bot such that it posts a comment like this only when auto-merge is blocked for exceptional, non-intuitive reasons. When the bot's auto-merge capability is properly configured, auto-merge should operate as you would intuitively expect and you should not see any spurious comments.

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[smitpatel]

couldn't resist seeing that all other classes put IReadOnly* before IMutableAnnotatable.

[AndriySvyryd]

Will fix in a later PR