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.
Also, assume that unknown inputs to ldfld are temporaries, not unmanaged pointers.
This avoids emitting weird pointer casts when accessing fields on unresolved value types.
While support for multi-module assemblies isn't fully working yet; it is clear at this point that we want
to treat each module in a multi-module assembly separately for the purposes of the type system.
This is a work-in-progress; ILSpy is not functional with this commit.
The old code path still exists but is broken because some classes were modified for the new system.
The new system is still highly incomplete (types only have fields, but no methods).