Also took the opportunity to make the IsByRefLike flag not be a property
of the field layout (but use the newly approved custom attribute that
everyone else uses).
Fixes #4220.
With some changes I have in flight `GetFinalizer` in compiler traces
went from "might be worth looking into making this faster" to "we spend
15% of compilation time here".
The answer to `HasFinalizer` is cheap to cache and reduces the pressure
on `GetFinalizer` significantly.
