The plot thickens!

I thought relying on namePos was less useful if it's not reliable. I did not yet go looking for other places where name text position is calculated.

I tested this change (and also the other open 2.13.12 PRs https://github.com/scala/scala/milestone/101) in a customer codebase, it's all good.

The new warning showed up a couple of times, it seems good to have.

trait T { def f(): Int }
trait U { def f = 1 }

// now warns, both under joint and separate compilation. no warning in 2.13.11.
class C extends T with U

here's where both warnings appear in the community build: https://gist.github.com/SethTisue/a6ecf5a6934fd0535b61a0c1a2b233a7

The release thread makes it look a bit weird. My comment on release notes was going to be that people will expect the "migratory" warnings to be entirely consistent (but maybe Lukas missed one). Now I think that maybe "you broke my build by adding warnings but I still don't have coverage stats" may require mollifying remarks.