Removing accessors other than getter and setter can be ABI breaking. This
patch starts to formally include all accessor decls in the tree and diagnose
their removal. This change only applies to the ABI checker since we still
exclude accessors other than getter and setter when diagnosing source
compatibility.
Including accessors formally can also allow us to check the missing
of availability attributes for newly added accessors.
rdar://52063421
5c22fd50e2