[clang] [clang] Diagnose default arguments defined in different scopes (PR #124844)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

During offline discussion with Corentin he pointed out that [over.match.best]/4 talks about multiple declarations that are _found_, and not simply reachable, like my implementation currently assumes. I'll update it to take the results of name lookup into account.

https://github.com/llvm/llvm-project/pull/124844