Auto merge of #22530 - rprichard:master, r=dotdash

Fixes #22525

I wasn't sure if I should reuse `write::get_llvm_opt_level` or not.  It returns an `llvm::CodeGenOptLevel`, which is the Rust binding for `CodeGenOpt::Level`. `lto.rs` is passing an optlevel to LLVM's `PassManagerBuilder`, which takes an unsigned int.  `PassManagerBuilder`'s optlevel uses essentially the same enumeration (i.e. 0-3 with 2 as default), but it's implicit.

diff --cc src/librustc_trans/back/lto.rs
Simple merge