Cache completion validity, remove too-precise completion this inference

[weswigham]

Fixes #23285 some. Could still stand to be better.

From my comment in the other thread

Hm, you're right, Opened it up and tried it again and saw the problem. Anyway, I have a change that cuts it from around 8s to 3s on my machine; but the core of the issue is what isValidPropertyAccessForCompletions is doing right now - it's essentially doing a (partial) call resolution, which is fairly expensive. My change caches the result, so it can be reused if encountered again, which seems to help, since completions is always followed by a completion entry details request which currently repeats the work.

This fixes the obvious deficiencies we had with respect to repeated work and mostly unnecessary work - something in lodash is still hideously expensive to compare, however, which probably merits looking into separately.