Fix GC write barrier when writing async method return into continuation object

[BrzVlad]

Previous code was only checking if the type contains GC refs, which means it would fail to add a write barrier if the method returns an object without refs.

Fixes System.Text.Json.Tests random GC crashes

[Copilot]

Pull request overview

Fixes missing GC write barriers when an interpreted async method returns a reference type whose instance contains no GC pointers (e.g., System.Object, string), but the return value itself is still an object reference that must be stored with a write barrier.

Changes:

• Treat non-value-type async returns as object references and store them via SetObjectReference (write barrier).
• Keep memmoveGCRefs for value-type returns that contain embedded GC references; otherwise use memcpyNoGCRefs.