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

[Naman Dixit via cfe-commits]

namandixit wrote:

Hi, is it possible to add a separate warning for unknown prefixes (as requested in the issue)? Enabling `-Wpedantic` is usually considered a best practice, and mixing useful warnings (unknown attribute) and less useful ones (unknown attribute prefix) will only drive people to either disable the pedantic mode or disable the warning altogether (which is something that can already be done).

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