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

[Tomas Matheson via llvm-commits]

tmatheson-arm wrote:

These changes seem to be contrary to the [LLVM Coding Standards style recommendations](https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly). Specifically variable and function names should start with a lower case letter.

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