Unless checking accesses, do not include unique symbols or numbers in index types

[weswigham]

Fixes the issue of keyof now returning unique symbols that @kpdonn reported in #20721, and @ahejlsberg were you looking at another issue this fixed?

Regarding the implementation - I actually had to add a new relationship (ew) to handle comparing with constraints of generic index types. (To continue to allow, eg, T[typeof sym]).

[kpdonn]

FWIW I actually think it makes a lot of sense for keyof to include unique symbol. See #20721 for my reasoning, but I was only pointing it out because it seemed pretty clearly unintentional and not because I thought it was actually a problem.

If it made it through a release with nobody complaining maybe it's a sign it was a good change? 🤷‍♂️

[weswigham]

@kpdonn keyof T can produce symbols, but keyof T is assignable to string - that's a bit of a pickle.

[weswigham]

Closed in favor of #23145.