Managed return value support for async calls

Several things were identified during #120303 for managed return values:
- The JIT associates the `CALL_INSTRUCTION` mapping with the task-returning IL call instead of with the result of with the `AsyncHelpers.Await`. Associating with `Await` is more suited for the debugger.
- Currently `CordbJITILFrame::GetReturnValueForILOffset` may return a bogus result when the callee suspends
- Implementing the support when the callee suspends will require larger changes -- the current approach reads the result from the return register at the call return instruction, but for suspended callees we need to read it from somewhere else on resumption