[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.

The issue names Clang's [[gnu::abi_tag]] handling and Windows ABI behavior but no files or tests. Start by searching the Clang sources and tests for gnu::abi_tag and Windows ABI mangling; done means the attribute is no longer silently ignored, with either a diagnostic or ABI-tag mangling covered by tests.

Written by the indexing model from the issue text.