Auto merge of #130446 - durin42:llvm-20-fix-CommandLineArgs, r=workingjubilee

rustc_llvm: adapt to flattened CLI args in LLVM

This changed in
llvm/llvm-project@e190d07. I decided to stick with more duplication between the ifdef blocks to make the code easier to read for the next two years before we can plausibly drop LLVM 19.

`@rustbot` label: +llvm-main

try-job: x86_64-msvc

Author: bors