Bump vendored version of LibCST

Summary:
The new version has:
- some fixes to reduce edge cases that sometimes prevented
  us from picking up required imports (this should lead to a
  couple percent drop in reject rates)
- the ability to add `from __future__ import annotations`
  via a flag on the codemod instead of by picking it up from
  the stub file
- the ability to count how many annotations were actually added,
  which we could hook into to get quick feedback about how
  we are doing
- the ability to suppress all import statements if no annotations
  were added, which should greatly reduce the number of junk
  "dangling import" diffs that we reject.

I also discovered that LibCST doesn't actually have pyre support,
and as a result the `main` branch of LibCST had typecheck errors
due to type refinement mismatches and edge cases that never happen
but the type system picks up. I had to put up a new PR with fixes
to avoid getting pyre errors here:
Instagram/LibCST#542

Reviewed By: grievejia

Differential Revision: D32053591

fbshipit-source-id: 913631b3a8e76490e187f1a5d22ffae36bc4e610