[llvm] [workflows] Create a more descriptive title and body when creating a PR for backports (PR #80396)

[Tom Stellard via llvm-commits]

tstellar wrote:

I think the pull request title is the most important part since that is what shows up in email subjects.  So I added the release branch name to the title so it's clear that it's a backport.

https://github.com/llvm/llvm-project/pull/80396