[GitHub] Move Copilot instructions for LLVM (#165763)
GitHub allows specifying custom instructions for the GitHub Copilot
reviewer [1]. Currently, we have a top level file, but GitHub supports
having different instructions for different files, which requires
creating an `instructions` subdirectory with multiple files and a patch
it applies to.
This PR moves the top level file into a new `instructions` directory,
and make it apply to the `llvm/` subdirectory. I spoke with Mircea at
the Dev Meeting and that should match his original intent.
[1]
https://docs.github.com/en/copilot/how-tos/use-copilot-agents/request-a-code-review/use-code-review#customizing-copilots-reviews-with-custom-instructions
