Upgrade to Babel 7.12.6

Summary:
Monthly Babel update. As usual, I wait until there are multiple point releases because the Babel team tends to always break a lot of things in the first ~3 patch releases.

Release Notes: https://github.com/babel/babel/releases from 7.12.0 -> 7.12.6 inclusive. From a check of each change, nothing immediately jumps out as something that would impact us.

Changelog: [Internal]

Reviewed By: MichaReiser

Differential Revision: D24723845

fbshipit-source-id: 7db8daa119a4e34cd1fd4f61e7a635fcaf4e4a91