[Clang] [[gnu::abi_tag]] silently accepted on Windows

Currently Clang doesn't diagnose the use of `[[gnu::abi_tag]]` when targeting the Windows ABI, even though it's ignored. Instead, Clang should either issue a diagnostic that it's an unsupported attribute, or, IMO preferably, actually mangle the ABI tag into the function name. If the ABI tag isn't desired on Windows, that attribute can simply be removed after all to get the old behaviour.
