Add YieldAwaiter support to the async method builder delegate optimization (#17441)

Add YieldAwaiter support to the async method builder delegate optimization (#17441)

We added an optimization to async methods that lets the builder avoid allocating the Action delegate when it recognizes the awaiter being used.  Previously this was enabled for all of the publicly exposed awaiters in corelib, with the exception of YieldAwaiter (what's used with Task.YIeld).  It was enabled by having the awaiter implement an internal interface.  This commit just generalizes that interface name and then implements it on YIeldAwaiter to complete the picture.