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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> https://github.com/amykhuang/llvm-project/tree/attr-plugin
> This branch is [2 commits ahead](https://github.com/amykhuang/llvm-project/compare/llvm:llvm-project:main...attr-plugin), [1684 commits behind](https://github.com/amykhuang/llvm-project/compare/attr-plugin...llvm:llvm-project:main) llvm:main.

Yeah, this branch is missing all November commits, include aforementioned fix.

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