[clang] Fix to attribute plugins reaching an unreachable (PR #70877)

[Louis Dionne via cfe-commits]

ldionne wrote:

This is weird, it's the first time I see this and I couldn't find any obvious offending commit. It looks like it shouldn't be related, but at the same time we're not seeing this issue in any of the other builds in the Clang pipeline. You could consider updating the PR with the latest `main` content to re-trigger a CI run and see if that still fails.

https://github.com/llvm/llvm-project/pull/70877