Enable Contributions Through Pull-request For LLVM

-1 from me as well. GitHub UI feels extremely sluggish with large
projects, forks feel like needless hurdles to jump over,
especially for small patches - having to fork the entire monorepo,
just to do a GitHub PR would be extremely cumbersome.

Just recently I submitted a very tiny PR for PcapPlusPlus on GitHub
which involved:
1). Forking the repository
2). Cloning the fork
3). Applying the patch
4). Pushing it and creating a PR.
5). Deleting the fork (Since I didn't want it cluttering up my account
and I'm not a regular contributor to PcapPlusPlus).

Neither the pull request mechanism nor GitHub issues feel like they're
suitable for large projects since it's common to see large GitHub
projects with ignored issues and worse filtering/categorizing. And PRs
as a review mechanism are far worse than Phabricator or any other
dedicated code review software, and I think a lot of GitHub projects
use internal or separate code review tools as-is, with PRs being
second class citizens and are more prone to getting derailed. It's not
uncommon for GitHub issue or PR discussions to turn into massive
threads that quickly turn into bikeshedding and off-topic conversations.

It also feels like this will inevitably force people to use external
tools to handle it, instead of just being able to use Git on its
own (something like `git cl` comes to mind, which some may like and
some may not).

And Roman raised a lot of good points regarding other, more
bureaucratic aspects of GitHub and it raising the bar for existing
contributors.

Just my two cents in.