[clang] [Clang] show attribute namespace in diagnostics (PR #138519)

[Aaron Ballman via cfe-commits]

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

As far as the changes go, this LGTM. We can generalize in another PR (which will update some parts of this patch but not enough to be worried we're wasting effort).

https://github.com/llvm/llvm-project/pull/138519