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

[Mehdi Amini via llvm-commits]

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joker-eph wrote:

If we name this `git-llvm-push` then installing it in the path will allow usage as `git llvm-push`.

One can still rename the file locally, but standardizing it would help with the doc.

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