[mlir] Allow adding extra class declarations to interfaces.

Summary: This matches the similar feature on operation definitions.

Reviewers: jpienaar, antiagainst

Reviewed By: jpienaar, antiagainst

Subscribers: mehdi_amini, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74438