[mlir][Pass] Deprecate FunctionPass in favor of OperationPass<FuncOp>

The only benefit of FunctionPass is that it filters out function
declarations. This isn't enough to justify carrying it around, as we can
simplify filter out declarations when necessary within the pass. We can
also explore with better scheduling primitives to filter out declarations
at the pipeline level in the future.

The definition of FunctionPass is left intact for now to allow time for downstream
users to migrate.

Differential Revision: https://reviews.llvm.org/D117182