Auto merge of #76332 - Mark-Simulacrum:bootstrap-llvm, r=pietroalbini

Auto merge of #76332 - Mark-Simulacrum:bootstrap-llvm, r=pietroalbini

Add rust-dev component to support rustc development

This is preparatory work for permitting rustc developers to use CI-built LLVM rather than building it locally. Unlike distro-built LLVM, CI built LLVM is essentially guaranteed to behave perfectly for local development -- it is fully up to date, and carries all necessary patches.

This is a separate PR from #76349 because it needs to land before that one, since we want a master build with the full CI LLVM to be available for easier testing.