[llvm] [llvm][utils] Add a script to use PRs over pushing to main (PR #166473)

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph approved this pull request.

LG, let's mature this in tree, and move the doc to the LLVM development guide if/when we make PR mandatory and this replaces the current direct pushes to main.

https://github.com/llvm/llvm-project/pull/166473