Need to spec the behavior where async spilling cannot spill ref-like types

[VSadov]

This is a bit of a corner case.

Due to stack-only nature of ref-like types, they cannot be stack-spilled. The situations where stack spilling is necessary are uncommon and somewhat hard to define. We must however give an error.

This is not a fundamentally new scenario. The limitation already existed for types such as TypedReference.

We must:

• codify behavior in the language or compiler spec, even if broadly
• consider that this is an emit-time diagnostics.
  The effects of that:
  -- error is reported on "Build" only
  -- error does not change lambda inference.