EnC: disallow editing async methods with incompatible constraints on their method builders

Follow up on discussion: https://github.com/dotnet/roslyn/discussions/51189

In debug builds we emit async state machine types as classes so that we can add fields to them as needed during EnC. Emitting the state machine type as reference type instead of a value type might however conflict with a constraint:

If

1. the return type of an async method specifies a custom builder B via AsyncMethodBuilder, and
2. B.Start<TStateMachine>(ref TStateMachine), B.AwaitOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine), or B.AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine) have a constraint on TStateMachine that disallows it to be a reference type.

Fix: The compiler should identify these cases and keep the state machine type as struct. The IDE should disallow editing such async methods.