llvmPackages: Add an update script
llvmPackages: Add an update script

This is based on a combination of Gabor Greif's update script [0] and my
script to verify the hashes [1]. It's a bit hacky / unconventional but
it should be very useful and worked fine so far (but it might require
changes for some edge-cases).

[0]: https://github.com/NixOS/nixpkgs/issues/100725#issuecomment-710055349
[1]: https://github.com/NixOS/nixpkgs/issues/100725#issuecomment-781323979

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