[Concurrency] Add an experimental macro for wrapping a function body in a new task.

[hborla]

It's very common to wrap an entire function body in a new task, e.g. in a synchronous event handler that needs to kick off async work. This change adds an experimental function body macro that wraps an entire function body in a task.

This macro is a much simpler version of the @asyncHandler attribute, which was proposed in an early pitch of SE-0297: Concurrency Interoperability with Objective-C, and subsetted out because many event handlers need to start work synchronously. This macro currently uses Task.init, but could be updated to use the pitched Task.startSynchronously API.

This macro is gated behind the ConcurrencySyntaxSugar experimental feature. I'll write up a pitch for this soon to discuss the design. Please save any design feedback for the forthcoming forum thread.

I'm currently working on lifting the limitation that function body macros cannot be applied to closures, so this macro can be used with closures too. There are quite a few places that assume an attached macro can only be applied to a declaration :)

[hborla]

@swift-ci please smoke test

[ktoso]

Naming bikeshed aside I think we'd probably want to adopt the name: (once #79600 is merged) and maybe priority: parameters on this as well 🤔

[hborla]

@swift-ci please smoke test

[hborla]

@ktoso ah yeah good idea! I'll wait for #79600 so I can add the parameters together.