[llvm] [RFC][TableGen] Restructure TableGen Source (PR #80847)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> Most of the diff seems to just clang-format changes

Indeed, it seems like git clang-format formatted the whole files when I moved them.
There's a lot of include changes though, all of the backends include their helpers using `Common/` or `Basic/`. Formatting was needed to sort the includes again

https://github.com/llvm/llvm-project/pull/80847