Auto merge of #77756 - alarsyo:setup-llvm-detect, r=jyn514

Detect configuration for LLVM during setup

This is a first draft to address #77579, setting `download-ci-llvm` to true on Linux, but I could also implement the `if-available` setting mentioned in the issue.

On other platforms I was thinking about using [the which crate](https://crates.io/crates/which), if adding a dependency on it is considered okay of course, to detect the presence of `llvm-config` in the path, and use it if found. Still a work in progress of course.