"Them's some strikingly lawyerly semantics."
The law explicitly makes a difference between consistent additional terms/amendments and new agreements :)
So i'm not shocked you find it this way, but it is what it is ...
Basically, there is a difference between having an agreement that happens to be spread out over multiple things, and two separate agreements.
If you next try to ask where that line is, without a complete integration clause, the answer is basically ¯\_(ツ)_/¯ without going to court. You will get different answers from different folks.
"If I were to release a work without a license (governed by the default of copyright law), and then explicitly grant a few new rights that don't exist in the absence of that "set of rights", non-lawyers might be inclined to think of this as a "new license"."
Well, this is a slightly different case. In this case, you had no rights at all, and you are giving rights for the first time. This probably semantically would be viewed as a new license.
In the LLVM case, there is a license agreement. It is being supplemented :)
"Speaking of that, what's the extent of case law for open-source licenses? Is there a compilation somewhere of decisions that make for good background reading?
"
Sadly, no.
There's basically only one or two decisions you can find at all.
The rest is basically viewpoints based on contract law.