Fixed contextual async return type with unions in context that contain a mix of promise and non-promise types

[Andarist]

Please verify that:

• There is an associated issue in the Backlog milestone (required)
• Code is up-to-date with the main branch
• You've successfully run gulp runtests locally
• There are new or updated unit tests validating the change

This potentially fixes #47682

Commits:

• first commit just add a test case to showcase the weird behavior
• second commit fixes the thing in one-line change
• third commit just refactors the code to avoid fallthrough and to make all paths more distinct and easier to follow (in my opinion), this is a subjective matter and I can revert this if needed

[Andarist]

The main and only change here is that for async non-generator functions I use mapType with getAwaitedTypeOfPromise. This ensures that I only unwrap promise types instead of unwrapping any types with Awaited<T>.

It IMHO conceptually makes sense because if we deal with unions we can only return a Promise type that includes Promise-wrapped types. If a member of a union is of a different type then async method just won't be able to match it as async always returns a Promise.

[Andarist]

@rbuckton friendly 🏓 would love to hear out your thoughts about this PR :)

[Azarattum]

@Andarist would this fix #48927?

[Andarist]

@Azarattum I’ve checked and those are unrelated

[Andarist]

@rbuckton friendly 🏓

[sandersn]

since #51196 supersedes this PR, do you want to close it?

[Andarist]

Ye, I think that this other PR is a better one. Let's close this one.