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

[Amy Huang via cfe-commits]

amykhuang wrote:

Updated the PR description and the patch. The existing `clang/test/Frontend/plugin-attribute.cpp` seems sufficient for test coverage. (I guess it wasn't being tested on bots though, otherwise we would have seen it?)

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