[Stacked 2/3] Prevent stale managed return values for Runtime Async suspension

[tommcdon]

Note

Stacked PR 2/3, based on dotnet/runtime#131985.
Next: tommcdon/runtime#2
Only the final MRV commit is new. Retarget to dotnet/runtime:main after the base PR merges.

Summary

Prevent debuggers from exposing stale managed return values when a Runtime Async call suspends.

The JIT reports the async continuation register alongside managed-return-value debug information. DBI rejects the value while that continuation is non-null, while preserving return values for synchronously completed calls. Interpreter async calls suppress MRV homes until equivalent continuation-state information is available.

Includes Windows and Linux coverage for synchronous completion and genuine suspension.

Partially addresses dotnet#121011.

[tommcdon]

Closing this fork-only stacked PR. The branch has been rebased independently onto dotnet/runtime main and will be submitted as a standalone upstream PR.