[mlir] Don't emit forward declaration for user defined storage classes

Currently DefGen::emitDecl always emits forward declarations of storage classes even for user define ones, which makes it difficult to use template class directly in ODS. This patch changes `DefGen` not to emit forward decl when `genStorageClass` is false.

Original discussion: https://discourse.llvm.org/t/use-template-classes-as-user-defined-storage-classes/72015

Reviewed By: mehdi_amini, rriddle

Differential Revision: https://reviews.llvm.org/D155225