[Clang] Enable MSVC embedded records for Plan 9

[ksaunders]

Summary

This diff enables MSVC embedded records when Plan 9 extensions are enabled. As well, it creates the Plan 9 warning group.

Please see the initial Plan 9 diff for more details about this extension and more: https://reviews.llvm.org/D127462.

Aside: It's not clear if Clang developers prefer these incremental diffs, or one big diff. Let me know.

Depends On: https://reviews.llvm.org/D127462

clang/include/clang/Basic/DiagnosticGroups.td

def GNU : DiagGroup<"gnu", [GNUAlignofExpression, GNUAnonymousStruct,
                             GNUZeroLineDirective,
                             GNUZeroVariadicMacroArguments]>;
 // A warning group for warnings about code that clang accepts but gcc doesn't.
 def GccCompat : DiagGroup<"gcc-compat">;
 
 // A warning group for warnings about code that may be incompatible on AIX.
 def AIXCompat : DiagGroup<"aix-compat">;
 
+// Warnings for Plan 9 extensions.
+def Plan9AnonTag : DiagGroup<"plan9-anon-tag">;
+
+// Warnings group for warnings about Plan 9 extensions.
+def Plan9 : DiagGroup<"plan9", [Plan9AnonTag]>;
+
 // Warnings for Microsoft extensions.
 def MicrosoftCharize : DiagGroup<"microsoft-charize">;
 def MicrosoftDrectveSection : DiagGroup<"microsoft-drectve-section">;
 def MicrosoftInclude : DiagGroup<"microsoft-include">;
 def MicrosoftCppMacro : DiagGroup<"microsoft-cpp-macro">;
 def MicrosoftFixedEnum : DiagGroup<"microsoft-fixed-enum">;
 def MicrosoftSealed : DiagGroup<"microsoft-sealed">;
 def MicrosoftAbstract : DiagGroup<"microsoft-abstract">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_anonymous_record_bad_member : Error<
   "anonymous %select{struct|union}0 can only contain non-static data members">;
 def err_anonymous_record_nonpublic_member : Error<
   "anonymous %select{struct|union}0 cannot contain a "
   "%select{private|protected}1 data member">;
 def ext_ms_anonymous_record : ExtWarn<
   "anonymous %select{structs|unions}0 are a Microsoft extension">,
   InGroup<MicrosoftAnonTag>;
+def ext_plan9_anonymous_record : ExtWarn<
+  "anonymous %select{structs|unions}0 are a Plan 9 extension">,
+  InGroup<Plan9AnonTag>;
 
 // C++ local classes
 def err_reference_to_local_in_enclosing_context : Error<
   "reference to local %select{variable|binding}1 %0 declared in enclosing "
   "%select{%3|block literal|lambda expression|context}2">;
 
 def err_static_data_member_not_allowed_in_local_class : Error<
   "static data member %0 not allowed in local %sub{select_tag_type_kind}2 %1">;

clang/lib/Sema/SemaDecl.cpp

if ((Tag && Tag->getDeclName()) ||
       if (Tag)
         Record = dyn_cast<RecordDecl>(Tag);
       else if (const RecordType *RT =
                    DS.getRepAsType().get()->getAsStructureType())
         Record = RT->getDecl();
       else if (const RecordType *UT = DS.getRepAsType().get()->getAsUnionType())
         Record = UT->getDecl();
 
-      if (Record && getLangOpts().MicrosoftExt) {
-        Diag(DS.getBeginLoc(), diag::ext_ms_anonymous_record)
+      if (Record && (getLangOpts().MicrosoftExt || getLangOpts().Plan9)) {
+        Diag(DS.getBeginLoc(), getLangOpts().MicrosoftExt
+                                   ? diag::ext_ms_anonymous_record
+                                   : diag::ext_plan9_anonymous_record)
             << Record->isUnion() << DS.getSourceRange();
         return BuildMicrosoftCAnonymousStruct(S, DS, Record);
       }
 
       DeclaresAnything = false;
     }
   }
 

clang/test/Sema/2c-anon-record.c

+// RUN: %clang_cc1 %s -fsyntax-only -Wplan9 -verify -fplan9-extensions
+
+struct A {
+  int x;
+};
+
+struct B {
+  struct A; // expected-warning {{anonymous structs are a Plan 9 extension}}
+};