Rollup merge of #123642 - onur-ozkan:restrict-llvm-option, r=Mark-Simulacrum

do not allow using local llvm while using rustc from ci

From: rust-lang/rust#123586 (comment)

> Even if `llvm.download-ci-llvm` is set to true, `stage > 0` rustc will always use the prebuilt LLVM library which comes with ci-rustc. So I tried to use locally-built LLVM libraries in the ci-rustc by replacing the existing LLVM libraries with the locally built ones, and it appears that this is indeed a limitation of using `rust.download-rustc=true` as it fails with the following error:
>
> ```
> $ ./build/host/ci-rustc/bin/rustc --version
> ./build/host/ci-rustc/bin/rustc: symbol lookup error: /home/nimda/devspace/.other/rustc-builds/build/x86_64-unknown-linux-gnu/ci-rustc/bin/../lib/librustc_driver-a03ea465d8e03db1.so: undefined symbol: LLVMInitializeARMTargetInfo, version LLVM_18.1
> ```
>
> So, if `rust.download-rustc` is set to true and `llvm.download-ci-llvm` is false, I believe bootstrap should terminate the process (as it always uses prebuilt LLVM libraries from ci-rustc, there is no point to build LLVM locally) while parsing the configuration.

Resolves #123586

r? Mark-Simulacrum

Author: matthiaskrgr