[llvm] [CI] Make issue labeler more robust (PR #145431)

[Prajwal Nadig via llvm-commits]

snprajwal wrote:

I understand the reasoning - it's just that there might be cases where someone goes through the link, discards the issue, and files it later (e.g. waiting for a PR to be merged before submitting the request). I personally found it easier to just type the issue title out instead of navigating the Developer Policy and clicking again, hence the change to retain the same regex but case-insensitive. We can also close this if you are sure you do not want the labeler to handle manually typed requests.

https://github.com/llvm/llvm-project/pull/145431