Rollup merge of #163003 - Zalathar:llvm-backend, r=Walnut356

cg_llvm: Move all code out of the crate root

Having non-trivial code in the crate root causes a few problems, most of which stem from every item being effectively `pub(crate)`:

- Fields/methods can't be private, so enforcing abstractions is impossible
- Every `use` becomes a de-facto re-export, leading to messy imports
- Moving items out of the root requires visibility adjustment

This PR therefore moves all non-trivial code out of the crate root, and into `rustc_codegen_llvm::back::llvm_backend`.

---

Deciding where to move the code *to* was a bit tricky. But the code being moved delegates a lot of its methods to other submodules in `back`, so creating another submodule in `back` seemed reasonable. I certainly like it more than the status-quo.

The main commit doesn't modify any of the code being moved, other than updating imports, as verified manually by:

```shell
git diff --color-moved --color-moved-ws=allow-indentation-change head^ head
```

There should be no change to compiler behaviour.