[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:

> From my understanding of the issue, an _unknown_ attr scope shouldn't result in a warning by default. It should only be handled when a specific flag is enabled or in pedantic mode. For that reason, I added an extension warning., or is this not applicable in this case?

That is not my understanding.  For example, we'd want to warn/identify someone typing `gcc` instead of `gnu`, or `calng` instead of `clang`, the same way we want to identify `nodiscard` vs `no_discard`.  

Did you see a discussion about this somewhere else?  I'd be curious to see what @AaronBallman has to say about this when he returns from his break.

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