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

[Erich Keane via cfe-commits]

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@@ -179,6 +179,8 @@ def err_opencl_unknown_type_specifier : Error<
 
 def warn_unknown_attribute_ignored : Warning<
   "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
+def ext_unknown_attribute_ignored : Extension<
+  "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
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erichkeane wrote:

>Do we want to support a grouping like -Wno-unknown-attribute-namespace=frobble where leaving off the = means we silence all unknown attribute namespaces?

I think that is a NEAT feature, but I fear one that is a lot of work for something that basically no one will figure out how to use (since people discover -Wno flags from our diagnostics), and thus no one will. If we DO something like that, I would want to see if we could do a note attached to it of:
`to disable this diagnostic for this namespace, use `-Wno-unknown-attribute-namespace=frobble`.  ALSO, we should support comma-delimited.

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