TODO:

• Bump LKG to get Preserve alias symbols on references to type aliases via imports #51152 and Fix inferences between alias type arguments and defaulted alias type arguments #51771 (fixing the errors currently in this PR).
• Bump LKG to get Improve logic that chooses co- vs. contra-variant inferences #52123 and Tweak co/contra inference logic from #52123  #52180
• Remove newly-added explicit type parameters fixed by the previous PRs
• Decide the relationship between TIn and TOut in the new visitor API; this actually mirrors the problems I have with cast in Sub-optimal type parameter inference with strictFunctionTypes enabled #52111, but to an even greater extent.
  • My personal feeling is that the PR is correct as-is; a visitor can return a node of a type unrelated to the input type so long as it's then used properly. E.g. if a node has a prop of type Identifier, but accepts Expression, it should be fine to visit that and return a different expression and assign it, even when the input might be known to be Identifier explicitly.
  • It turns out that there is code which requires that the input not be related to the output (in the same manner I speculated about above), so, the code needs to stay as-is.
• Decide whether or not visitor should still be optional.
• Maybe deal with createJSDocDeprecatedTag.
• Maybe deal with isParameter.
• Decide if we are okay exporting BufferEncoding.
• Decide VisitResult<Node | undefined> or VisitResult<Node> | undefined?
• Maybe fix private handlers = new Map(...)
  • I investigated this; it turns out that a lot of our handlers don't actually match the type they're declared to return in the protocol! This turns into a much larger change so I think it's best to leave to another PR.
• Add this to https://github.com/microsoft/TypeScript/wiki/API-Breaking-Changes.