New Herald rules to ease libc++ and libc++abi reviews

Folks,

We've been operating reviews in a way that requires new committers to know what individuals to ping to get a review in libc++ and libc++abi, and also makes it unclear what the bar for acceptance is. I felt that improving on this front would make it easier for new committers to get involved (they don't need to fill the Reviewers: field anymore), and also would resolve some conflicts that can arise when people are unsure what approvals they need.

For this reason, I went ahead and asked for the creation of the libc++ and libc++abi Projects in Phabricator (thanks Ben Hamilton). These projects will automatically be added as Blocking reviewers on changes that touch files in libcxx/* and libcxxabi/*, respectively. The idea is that when any member of one of these groups approves the review, the review can move along. Obviously, if the opinion of a particular person is requested during the review, that should override everything else, but I believe this can be handled on a case by case basis. That should make it painless for everyone to meet our review expectations, including new contributors or those simply less familiar with our review process.

These review groups are not closed -- they allow self-add. I believe the code owners for libc++ are known (usually Marshall, EricWF or myself), but if you consider yourself a code owner of libc++abi, please go ahead and add yourself to the group. Keep in mind that if you vet some changes and they break something, as a code owner it is your responsibility to do something about it if the contributor isn't in a position to.

I believe this change should help model the current review reality for libc++ and libc++abi -- which was my primary goal.

Cheers,
Louis

P.S.: I haven't done anything about libunwind, since I haven't been involved in that project nearly as much, and I don't consider myself a code owner for it. If there's a desire to setup the same system for libunwind, just let me know, it's easy to do.