Support moving continuations to SynchronizationContext/TaskScheduler

[jakobbotsch]

This adds support for moving continuations after task awaits to the right context:

• For unconfigured task awaits or ConfigureAwait(true), continuations are posted to the captured SynchronizationContext or TaskScheduler by the BCL
• For ConfigureAwait(false) continuations are moved to the thread pool

The JIT inserts a call to a new AsyncHelpers.CaptureContinuationContext when suspending because of a task await. That function either captures the current SynchronizationContext or TaskScheduler into the continuation in a known place. Later the BCL will fetch the context from the continuation object based on a flag to determine whether the continuation needs to be posted somewhere else or whether it can be inlined. There is a new Task-derived ThunkTask that replaces the existing C#-async-function-with-a-loop and which handles the continuations explicitly.

Inlining of continuations currently has no check on stack depth. This might be necessary to add. There are some differences compared to async 1 with how continuations are executed. In particular some common cases that resulted in unbounded stack usage with async 1 does not result in increasing stack usage with runtime async. Hence I am not 100% sure whether we need a mitigation or not, but more investigation is needed from my side. I'd like to leave that as a follow-up.

To get to the right behavior I have disabled inlining of task-awaited functions. This is a large concession, but getting the behavior right in the face of inlining proved to be quite tricky with the JIT's current internal representation. It is near top priority for me to reenable the inlining support, but I would like to work on it separately.

There is still one missing piece for correctness: saving and restoring the Thread._synchronizationContext field around async calls. It runs into similar trickiness when trying to represent the expected behavior (which is that the field is restored around synchronous calls, but not restored on resumption).

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