[llvm] workflows: Add a simple pull request subscription workflow (PR #64913)

[Tom Stellard via llvm-commits]

tstellar wrote:

I rewrote this so now labels are added to PRs based on the files changed by the PR, and then teams are notified with an @ mention based on the labels.  I also added a diff of the PR to go along with the mention, so subscribers can see the context in their email without having to click on the PR link.

Here is an [example](https://github.com/llvm/temp-issue-tester/pull/53#issuecomment-1711047749) of what the comment/notification will look like.

https://github.com/llvm/llvm-project/pull/64913