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

[Tom Stellard via llvm-commits]

tstellar wrote:

@linux4life798 I was able to get it to work by increasing the cache size to 4GB: https://github.com/tstellar/llvm-project/actions/runs/7618766244/job/20750565953

I'm going to try to see how low I can make it while it still works.

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