[clang] [clang] Diagnose when spelled 'abi_tag' on namespace doesn't match applied one. (PR #221039)

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

Thanks, LGTM: the dedicated check removes the attribute-inheritance side effects, subset reopenings are quiet, and the message lists only the dropped tags.

https://github.com/llvm/llvm-project/pull/221039