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

[Oleksandr T. via cfe-commits]

================
@@ -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">,
----------------
a-tarasyuk wrote:

@AaronBallman thanks for the feedback. There is a test that disallows such options from appearing in the help - should it be changed?

https://github.com/llvm/llvm-project/blob/ac94fade6075fec89eb29c7dedf01ef59601e61d/clang/test/Driver/immediate-options.c#L6

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