Actually LLVM has their official repository there, not a mirror as GCC[0]. But they still do reviews and merges outside of github. linux kernel project does a similar thing.
The "linux kernel project" does not have its official tree on github, Linus just has a read-only mirror there.
You can confirm this by reading one of the bot replies to a PR [0]:
Linux kernel development happens on mailing lists, rather than on GitHub - this GitHub repository is a read-only mirror that isn't used for accepting contributions. So that your change can become part of Linux, please email it to us as a patch.
I know that the discussions and reviews happen on mailing lists and believed that this github repo is the official one there all these patches go, but apparently it's not.
You can confirm this by reading one of the bot replies to a PR [0]:
Linux kernel development happens on mailing lists, rather than on GitHub - this GitHub repository is a read-only mirror that isn't used for accepting contributions. So that your change can become part of Linux, please email it to us as a patch.
[0] https://github.com/torvalds/linux/pull/805#issuecomment-5933...