Remember fresh type variables for unions and await statement

Remember fresh type variables for unions and await statement

Summary:
More replacement of `fresh_type` and `fresh_unresolved_type` with versions that record the type variables generated.

* We sometimes generate unions by first generating a fresh variable and then do subtyping against it (`compute_supertype`).
* For `yield` and `yield from` expressions
* For signatures generated for `gena` etc.

Reviewed By: CatherineGasnier

Differential Revision: D13254586

fbshipit-source-id: d4e9c282a44362b803a6ba284390e39443a1dea5