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

[Jason Eckhardt via llvm-commits]

nvjle 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.

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