Experiment with removing `originalKeywordKind` from every `Identifier`

[DanielRosenwasser]

In our factories, every Identifier has an unconditional assignment to originalKeywordKind. This feels wasteful, but it's at the expense of reducing polymorphism. As an experiment, we should see

1. Whether we can conditionally set it at no cost, or
2. Whether we can just remove it entirely, computing the necessary information on the fly.

[DanielRosenwasser]

I think we shimmed this with a get-accessor.

[jakebailey]

Yep: https://github.com/microsoft/TypeScript/blob/main/src/deprecatedCompat/5.0/identifierProperties.ts#L28