Verify that Heap::QObjectMethod::ensureMethodsCache works correctly

The logic in ensureMethodsCache makes certain assumptions about the layout of the methods in the meta-object, and the start index.

It is unclear whether those truly hold, especially when both public and private methods exist in the overload set.

We should investigate it, and

• either fix the existing logic
• or, if it is correct, add more internal documentation on how it works