[update-checkout] make llvm-project be the first project

[charles-zablit]

Cloning llvm-project is always the limiting factor when cloning from an empty directory with update-checkout, as it is the largest repository. If it's not one the first repositories being cloned, the runtime for the script increases. This is even more apparent on machines with a lower number of cores.

This patch ensures that llvm-project will always be the first repository to get processed when running update-checkout.

Testing locally on a MacBook Pro M4 Pro with 28 processes gives a runtime improvement between 14% and 18%.
./swift/utils/update-checkout --clean --reset-to-remote --clone --scheme main

With the patch: 442.90s user 174.85s system 192% cpu 5:20.23 total
Without the patch: 471.91s user 186.90s system 171% cpu 6:23.50 total

Please note that the benchmark should be repeated multiple times to get more accurate results, as this is also very much network dependent.

[charles-zablit]

@swift-ci please test