[mlir][OpDSL] Rename `PrimFn` to `ArithFn`.

The revision renames `PrimFn` to `ArithFn`. The name resembles the newly introduced arith dialect that implements most of the arithmetic functions. An exception are log/exp that are part of the math dialect.

Depends On D115239

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D115240