[llvm] [Github][RFC] Add workflow to diff codegen on llvm-test-suite (PR #190010)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Is it possible to refactor the CMake caches you're creating out into separate files?

Done in ba5f906c3b42df714931ba8515a519783dd3be86, but one thing to watch out for is that we need to do another checkout of the repository to make sure we use the scripts/cmake files in the upstream repository, not the PR which is untrusted: af377f061e65adf6b1777de5c4de5a1659feb4a4

The CodeQL workflow picked this up. But it still seems to complain about using the ./utils/tdiff.py scripts etc. in llvm-test-suite. I think that's a false positive though, since they're always from the main branch of llvm/llvm-test-suite and not e.g. the PR

https://github.com/llvm/llvm-project/pull/190010