[llvm] workflows: Merge LLVM tests together into a single job (PR #78877)

[Craig Hesling via llvm-commits]

linux4life798 wrote:

Also in terms of caching options, it looks like sccache has builtin support for Github ation cache. See https://github.com/mozilla/sccache/blob/main/docs/GHA.md.

https://github.com/llvm/llvm-project/pull/78877