[clang] [Frontend][Sema] Add CC1-only -fms-anonymous-structs to enable Microsoft anonymous struct/union feature (PR #176551)

[Erich Keane via cfe-commits]

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@@ -3327,6 +3327,12 @@ def fms_extensions : Flag<["-"], "fms-extensions">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption]>,
   HelpText<"Accept some non-standard constructs supported by the Microsoft compiler">,
   MarshallingInfoFlag<LangOpts<"MicrosoftExt">>, ImpliedByAnyOf<[fms_compatibility.KeyPath]>;
+def fms_anonymous_structs
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erichkeane wrote:

Yes, we should still have it negate-able.  We typically like to have that since we have a 'last one wins' sorta rule, which is important for folks who use build scripts.  

https://github.com/llvm/llvm-project/pull/176551