[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:

Curiously, and after I sent that, I realized why _WE_ of all compilers SHOULD warn here, even if others don't.  Clang supports/recognizes the prefixes of all of the major compilers, which drastically increases the chance that an unrecognized one is a typo rather than an intentional difference.  

It would be justifiable to have a 'list' of ones that we don't recognize, but acknowledge that is hidden under that extension warning, but we SHOULD be doing `gcc` and `gmu` and `clagn` the same severity/applicability as if you misspelled it as `caries_dependency`.

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