Rollup merge of #136881 - dpaoliello:cleanllvm3, r=Zalathar

cg_llvm: Reduce visibility of all functions in the llvm module

Next part of #135502

This reduces the visibility of all functions in the `llvm` module to `pub(crate)` and marks the `enzyme_ffi` modules with `#![expect(dead_code)]` (as previously discussed: <rust-lang/rust#135502 (comment)>).

r? ``@Zalathar``

Author: jhpratt