Update developer policy on potentially breaking changes

We've recently had issues appropriately notifying users and
stakeholders of changes to projects that may be potentially disruptive
when upgrading. This led to discussion on Discourse about how to
improve the situation, which can be found at:
https://discourse.llvm.org/t/rfc-add-new-discourse-channel-for-potentially-breaking-disruptive-changes-for-clang/65251

Ultimately, it sounds like we want to encourage three things:

* Alert vendors during the code review so they can provide early
  feedback on potentially breaking changes that would be unacceptable
  for them.
* Alert vendors and users after committing the changes so they can
  perform pre-release testing on a completed change to determine if it
  causes unreasonable problems for them.
* Alert users more clearly through the release notes so that it's
  easier to determine how disruptive an upgrade might be.

This updates the developer policy accordingly.

Differential Revision: https://reviews.llvm.org/D134878