move `download-ci-llvm` logic from bootstrap.py to rustbuild

[alarsyo]

As discussed a bit in the comments of #77756 , it'd be great if the logic handling LLVM setup / download was integrated with rustbuild directly. The main blocker being that rustbuild would need to depend on an http client library.

Currently, the python script does system calls to curl or to Powershell's WebClient depending on the platform.

[est31]

rustbuild doesn't have to depend on a http client library, it can just shell out to curl or WebClient as well. I think that's perfectly fine for anything for which http downloads are not the main purpose.

[Mark-Simulacrum]

It looks like Python is already doing that -- interesting -- in that case it definitely seems fine to do so. It would be great if we could avoid duplicating the curl invocations but I wouldn't bend over backwards to do so.

[jyn514]

Done in #95170.