cg_llvm: More preparation for moving FFI bindings to rustc_llvm

[Zalathar]

• Sequel to cg_llvm: Remove inherent methods from several LLVM FFI types #147327
• Motivated by Move all LLVM externs into the rustc_llvm crate #142897

---

This is another batch of adjustments to code that would otherwise interfere with moving FFI bindings out of rustc_codegen_llvm and into rustc_llvm.

[rustbot]

r? @SparrowLii

rustbot has assigned @SparrowLii.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer