Relax enforcement of conditional attributes

[jcouv]

class C<T>
{
    bool Method([MaybeNullWhen(false)] out T value)
    {
        ...
        return someBoolLocal; // will warn and is hard to suppress
    }
}

Should we relax the enforcement (since we don't track bool locals), or should we offer a way of suppressing?

Relates to dotnet/runtime#36104 (comment)

Options so far:

1. assertions don’t work as far as I can tell
   Even making the two explicit branches of the return explicit with an if (someBoolLocal) { someAssertion(); return true; } else { return false; } there are no good assertions that resolve this.

2. suppression with !
   You could bang all the assignments to value. Seems smelly though

3. explicit suppression
   #pragma warning disable CS8762

4. allow silencing the return statement
   return someBoolLocal !;.

5. Only warn when we know the value of someBoolLocal.

[jcouv]

We'll go with option 5