[mlir] Move SymbolOpInterfaces "classof" check to a proper "extraClassOf" interface field

SymbolOpInterface overrides the base classof to provide support
for optionally implementing the interface. This is currently placed
in the extraClassDeclarations, but that is kind of awkard given that
it requires underlying knowledge of how the base classof is implemented.
This commit adds a proper "extraClassOf" field to allow interfaces to
implement this, which abstracts away the default classof logic.

Differential Revision: https://reviews.llvm.org/D140197