[Concurrency] Remove _unsafeInheritExecutor from public APIs, use #isolation

[ktoso]

We should be using the #isolation macro in all APIs where _unsafeInheritExecutor was used before;

This handles it just for the continuation APIs, more to follow soon.

resolves rdar://125307764
resolves #69659

---

It seems it runs into availability issues though?

/Users/ktoso/code/swift-project/swift/stdlib/public/Concurrency/CheckedContinuation.swift:292:40: error: 'isolation()' is only available in macOS 10.15 or newer
╰─ /Users/ktoso/code/swift-project/swift/stdlib/public/Concurrency/CheckedContinuation.swift:292:39: note: expanded code originates here
//    @available(SwiftStdlib 5.1, *) <<<<<<< this is macOS 10.15 (SwiftStdlib 5.1:macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0)
//    @inlinable
//    @_unavailableInEmbedded
//    public func withCheckedContinuation<T>(
//      isolation: isolated (any Actor)? = #isolation,
//      function: String = #function,
290 │   _ body: (CheckedContinuation<T, Never>) -> Void
291 │ ) async -> T {
292 │   return await withUnsafeContinuation { <<<<<<< this now has #isolation
    ╭─── /Users/ktoso/code/swift-project/swift/stdlib/public/Concurrency/CheckedContinuation.swift
    │290 │
    │291 │
    │292 │                                       #isolation
    │    │                                        ╰─ error: 'isolation()' is only available in macOS 10.15 or newer
    ╰──────────────────────────────────────────────────────────────────────────────────────────────
293 │     body(CheckedContinuation(continuation: $0, function: function))
294 │   }

where the unsafe is:

@available(SwiftStdlib 5.1, *)
@_alwaysEmitIntoClient // removing this doesn't matter
public func withUnsafeContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  _ fn: (UnsafeContinuation<T, Never>) -> Void
) async -> T {
  return await Builtin.withUnsafeContinuation {
    fn(UnsafeContinuation<T, Never>($0))
  }
}

[ktoso]

Unblocked by #72813

[ktoso]

Now this still needs some work about backdeployed + unavailable in embedded I think, otherwise it'd be good to go

[ktoso]

@swift-ci please smoke test

[ktoso]

So it turns out solving this issue took a few fixes in related language features and quite some time, but the hackathon is where we kicked off the diagnosis and fixes, so I'd like to thank you @kntkymt for collaborating on this back then!

[ktoso]

@swift-ci please smoke test