[llvm] [CI] Switch to a single runtimes build (PR #131913)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> I'd love if we could keep a way to pre-commit test these other configurations optionally, off-by-default, by for example requiring the user to add certain tags to the PR.

It should be technically possible. The main thing is exchanging build artifacts between the runs so that we don't have to recompile too many things. Given the runtimes builds can be separate and we only need to package clang between jobs, I think it should be doable, it's just a matter of actually doing it/prioritization

https://github.com/llvm/llvm-project/pull/131913