[Concurrency] Fix await fix-it placement

[etcwilde]

This PR does some work around the diagnostics on await. The first issue is that the fix-it for await put the await in the wrong place. The issue also exists for the placement of try. The fix-it would put the await immediately before the call or access, but this doesn't work if that point is immediately after a binary operation.

e.g.

let _ = 32 + await asyncFunc()

fails to compile, despite that being where the fix-it suggests putting it.
That gets fixed in this PR. We do this by walking up the parent to an anchor point where we want to emit the await to. By keeping those in a map, we can effectively de-dupe them and map all of the warnings back to a given anchor.
I did run into some difficulties with finding a decent anchor in string interpolated expressions (see CheckEffectsCoverage::walkToAnchor). If we're in a string interpolation, I've found that we make it to the top of the expression and parent becomes a nullptr. The top of the expression for a string interpolation appears to always be a CallExpr, which you then pull out the args from to get a ParenExpr. The subExpr of that appears to be thing thing we're looking for. If this always holds true, we can remove the cascade of if's and just make those the appropriate casts. My concern with being too forceful is getting this assumption wrong and leading to a crashing compiler. As implemented now, it will just suggest that the await goes in a weird spot. e.g.

  print("\(32 + x.number)")
          ^~~~~~~~~~~~~~~
          await

instead of

  print("\(32 + x.number)")
           ^~~~~~~~~~~~~
           await

Additionally, instead of repeatedly emitting errors for every part of the expression, we emit one error with the necessary fix-it, and emit notes for each part that is the cause for needing to put the await in the right place.

e.g.

let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber

will now emit:

awaitPlacement.swift:20:11: error: expression is 'async' but is not marked with 'await'
  let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          await
awaitPlacement.swift:20:16: note: call is 'async'
  let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber
               ^
awaitPlacement.swift:20:29: note: call is 'async'
  let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber
                            ^
awaitPlacement.swift:20:42: note: property access is 'async'
  let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber
                                         ^
awaitPlacement.swift:20:67: note: property access is 'async'
  let _ = 32 + longTask() + longTask() + actorInstance.myNumber + actorInstance.myNumber
                                                                  ^

Will need to eventually fix the fix-it for try, but given that this is new syntax, it's a bit more important to get right. Folks are probably more familiar with where a try goes by now. Maybe.

Fixes: rdar://76245239

[etcwilde]

@swift-ci please smoke test

[etcwilde]

@swift-ci please smoke test

[etcwilde]

@swift-ci please smoke test macOS

[ktoso]

Nice, looks good to me

[etcwilde]

I'm going to go ahead and merge this because updating the tests is a pain. If we need something changed, I can do that in a separate PR.