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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

> ping for review?

My apologies for the delay!

The changes LGTM but there is a precommit CI failure with libc++. As best I can tell, that looks to be a transient issue and not caused by your patch, but it would be good to verify that before landing. (I would be really surprised if this change managed to break the demangler behavior.) CC @ldionne @mordante @philnik777 for other opinions.

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