[mlir][OpDSL] Refactor function handling.

Prepare the OpDSL function handling to introduce more function classes. A follow up commit will split ArithFn into UnaryFn and BinaryFn. This revision prepares the split by adding a function kind enum to handle different function types using a single class on the various levels of the stack (for example, there is now one TensorFn and one ScalarFn).

Depends On D119718

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D120108