[llvm] [TableGen] [NFC] Refine TableGen code to comply with `clang-tidy` checks and remove unused imports (PR #113318)

[Tomas Matheson via llvm-commits]

tmatheson-arm wrote:

> > (@tmatheson-arm made a comment about this but I don't see it).
> 
> It was probably deleted by the author.

Yes sorry, I did comment but then upon double checking it looked like I was wrong, so I deleted the comment to try and avoid confusion.

https://github.com/llvm/llvm-project/pull/113318