[clang] [Clang] raise extension warning for unknown namespaced attributes (PR #120925)

[Oleksandr T. via cfe-commits]

a-tarasyuk wrote:

@namandixit it is possible. I'm not confident about the new option., @AaronBallman @erichkeane  - should _unknown namespaced attributes_ be separated by the new option?

https://github.com/llvm/llvm-project/pull/120925