Decouple `is_rust_llvm` from whether LLVM is in-tree or not

[jyn514]

The point of is_rust_llvm is to know whether it contains Rust patches. In #101072 @tmandry added a config option so you can use a vanilla upstream LLVM without affecting other parts of how bootstrap works, this breaks that by re-coupling the idea of "is patched" to "is in tree".

Originally posted by @tmandry in #101833 (comment)

[tmandry]

Thanks for filing this! #101072 is on beta, so

@rustbot label: -regression-from-stable-to-nightly

I guess we use milestones for issues like this?