[llvm] [TableGen] Emit OpName as an enum class instead of a namespace (PR #125313)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> The `Instruction Operand Name Mapping` section of `WritingAnLLVMBackend.rst` needs a corresponding minor update.

Right. I had that at the back of my mind, I started this as a prototype and then forgot to do it. Updated the documentation as well.


https://github.com/llvm/llvm-project/pull/125313