rustc_llvm: adapt to flattened CLI args in LLVM This changed in llvm/llvm-project@e190d074a0a77c9f8a7d7938a8187a7e2076e290. 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