Removal of rustc_codegen_utils breaks mdbook builds

[chrissimpkins]

It looks like the merge of rust-lang/rust#69965 led to mdbook build fails on Travis: https://travis-ci.com/github/rust-lang/rustc-dev-guide/builds/154502721#L342

It is due to this link to the librustc_codegen_utils docs in appendix/stupid-stats.md:

> NOTE: `librustc_trans` and `librustc_back` no longer exist, and we don't
> translate AST or HIR directly to LLVM IR anymore.  Instead, see
> [`librustc_codegen_llvm`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/index.html)
> and [`librustc_codegen_utils`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_utils/index.html).

The https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_utils/index.html URL lands on a 404 page. Should it be removed or is there an updated location that still pertains here?

[mark-i-m]

@chrissimpkins librustc_codgen_utils has been dissolved into a bunch of places. It probably makes more sense to point to librustc_codegen_ssa and librustc_codegen_llvm.

[chrissimpkins]

I'll push a PR with the updates shortly. Thanks Mark.

[chrissimpkins]

#630 should fix it