Potential badly-behaved coding patterns with runtime async

[jakobbotsch]

This issue captures some identified poorly performing coding patterns when runtime async is enabled, and some possible mitigations we should consider implementing.

• Common pattern of using synchronous Task-returning wrappers is currently a deoptimization ([runtime-async] Optimize synchronous Task-returning wrappers used in async context #115771)
  • Egor points out that Task.FromResult is common too
  • Issue identifies potential way to optimize this by aggressively inlining into runtime-async callable thunks.
• Large structs in runtime async may be repeatedly copied to and from heap if suspension/resumption is frequent (Support dynamically creating tailored continuation layouts #120411 (comment))
  • Potential fix probably involves PGO information to identify methods where suspension/resumption is common, or analysis of the size of continuations created

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