[RuntimeAsync] R2R and Native AOT support

[VSadov]

For R2R there are two parts here:

• R2R for Async helpers.
  We explicitly make them BypassReadyToRun, since R2R typesystem does not support MethodImpl.Async. THis could be a relatively easy fix.
• R2R for Async/Task variants of user-defined methods (those using user-provided IL).
  We exclude these in R2R. It is basically an NYI, that needs to be revisited, implemented, tested.
• R2R for Async/Task thunks.
  It is unclear whether this is beneficial. Needs to be revisited.

For NAOT, all code will need to be generated

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[VSadov]

One easy way to start is to make the managed type system familiar with IsAsync methods in IL. Recognizing that will be needed anyways.

IL emitter (i.e. Roslyn) marks new-style async methods with MethodImpl.Async, but right now only the native typesystem knows that it has a certain meaning.

[VSadov]

An interesting side-quest could be making ILSpy recognize async methods as well. Not sure if ILSpy uses the same type system or something related/similar.

Inside IsAsync method the body returns a value of an "unwarapped" formal return type. That is - if the formal type is a Task/ValueTask/Task<T>/ValueTask<T>, we expect the body to return an unwrapped value. No need to warn on ret instruction about return type mismatches, or underflows in a case of unwrapped type is void.

Also AsyncHelpers.Await(something) could be decompiled into await something