[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.
> 
> Also, this might be obvious, but if/when this lands, please cherry-pick into the NV-internal trybot since it won't merge cleanly. I'll do the same in a second backend I'm responsible for.

Yeah, I have them in an internal branch as well, ready to merge when this change (if committed) lands internally.

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