[llvm] [llvm][docs] Add notes on upstreaming code from downstream projects (PR #129743)

[Andrzej Warzyński via llvm-commits]

================
@@ -272,3 +272,19 @@ Developers should participate in code reviews as both reviewers and
 authors. If someone is kind enough to review your code, you should return the
 favor for someone else. Note that anyone is welcome to review and give feedback
 on a patch, but approval of patches should be consistent with the policy above.
+
+Upstreaming Changes to LLVM
+===========================
+
+When upstreaming your own changes from a downstream project to LLVM, simply
+follow the process outlined above.
+
+When upstreaming changes originally written by someone else:  
+
+* Ensure that the original author(s) are aware of and approve the upstreaming
+  of their code, and that they are comfortable with you leading the process.
+* Properly attribute the original changes, e.g., by creating a commit with
+  multiple authors ([GitHub guide](https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors)).  
+* Invite the original author(s) to review the changes, and also include
+  additional reviewers. Specifically, an LGTM from a (co-)author does not
+  constitute approval to land a change.
----------------
banach-space wrote:

> Is a code owner for a component or the lead for a specific platform (e.g., for Clang driver changes for that platform) unable to give approval to land a change if they developed part of it?

Wouldn’t this essentially equate to self-approving your own code?  

LLVM's developer policy already allows one to commit patches without review in certain cases:  

* https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access

> You are allowed to commit patches without approval which you think are obvious.  

When we do this, it's effectively _self-approval_, but the intent is clear - we typically justify direct pushes with reasons like *"trivial typo"* or *"fix broken bot to unblock people."*  

I’d prefer to avoid creating a new category of *"code-owner reviewing their own code submitted by another community member."* That seems like an unnecessary distinction and could blur the lines of our review process.


https://github.com/llvm/llvm-project/pull/129743