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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

To be honest, the diagnostic on the first declaration is not the common pattern. But in this case I think it is worth it.

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