ABI/API checker should check semantic differences of two modules.
Adhering too strictly to the actual ASTs could yield false positives. This
patch populates ObjC, Dynamic and Final to the attribute list if AST
APIs say so.
rdar://50217247
Under `-enable-library-evolution`, a slight AST difference is still found when importing
from .swiftinterface and from .swiftmodule. The difference is whether @objc attribute
is inherited by a Swift class subclassing an Objc class.