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

[Aaron Ballman via cfe-commits]

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@@ -1115,6 +1115,11 @@ defm cx_fortran_rules: BoolOptionWithoutMarshalling<"f", "cx-fortran-rules",
   NegFlag<SetFalse, [], [ClangOption, CC1Option], "Range reduction is disabled "
   "for complex arithmetic operations">>;
 
+def Wunknown_attribute_namespace_EQ : CommaJoined<["-"], "Wunknown-attribute-namespace=">,
+  Group<W_value_Group>, Flags<[HelpHidden]>, Visibility<[ClangOption, CC1Option]>,
+  HelpText<"Specify a comma-separated list of allowed unknown attribute namespaces">,
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AaronBallman wrote:

It's a little bit odd that we have help text for something with the `HelpHidden` flag. Can probably remove the `HelpText`.

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