Rollup merge of #101072 - tmandry:llvm-is-vanilla, r=Mark-Simulacrum

bootstrap: Add llvm-has-rust-patches target option

This is so you can check out an upstream commit in src/llvm-project and
have everything just work.

This simplifies the logic in `is_rust_llvm` a bit; it doesn't need to
check for download-ci-llvm because we would have already errored if both
that and llvm-config were specified on the host platform.