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

[Tom Stellard via llvm-commits]

tstellar wrote:

The macOS-11 builds have started to timeout now, because of how long it takes to install sccache, so I need to merge this in order to fix the workflows.  We can discuss optimal cache size or whether to switch to ccache in a follow up discussion.

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