If this attribute is in use, private/internal members lack nullability annotations.
Previously in such cases, we ended up inheriting the nullability from the `[NullableContext]`, which could cause us to display a misleading nullability for primary methods.
In debug builds, it could also trigger an assertion when trying to apply the "nullable reference type" marking to to value types.
Of note is that properties and events are a special case: they do not explicitly store Accessibility in metadata. For properties computing the accessibility requires decoding the signature (to find overridden base properties). So these two only check the declaring type's accessibility instead; private properties may still carry nullability despite `[NullablePublicOnly]`. However, the property accessors won't store nullability, so we need to read the `returnTypeAttributes` from the property itself.
Daniel Grunwald