[ASTImporter] Fix import failed when anonymous union defined in class

[jcsxky]

Summary

when import a class contains two anonymous unions, constructor accesses member in the second union would lead to import the second union, after that, import the first union will lead to conflict, skip when two union are both anonymous.

[danix800]

Already fixed by https://reviews.llvm.org/D154764 .

clang/lib/AST/ASTImporter.cpp

   auto FoundDecls = Importer.findDeclsInToCtx(DC, Name);
   for (auto *FoundDecl : FoundDecls) {
     if (FieldDecl *FoundField = dyn_cast<FieldDecl>(FoundDecl)) {
       // For anonymous fields, match up by index.
       if (!Name &&
           ASTImporter::getFieldIndex(D) !=
           ASTImporter::getFieldIndex(FoundField))
         continue;
-
+      if (D->isAnonymousStructOrUnion() && FoundField->isAnonymousStructOrUnion()) {
+        continue;
+      }
       if (Importer.IsStructurallyEquivalent(D->getType(),
                                             FoundField->getType())) {
         Importer.MapImported(D, FoundField);
         // In case of a FieldDecl of a ClassTemplateSpecializationDecl, the
         // initializer of a FieldDecl might not had been instantiated in the
         // "To" context.  However, the "From" context might instantiated that,
         // thus we have to merge that.
         // Note: `hasInClassInitializer()` is not the same as non-null

clang/unittests/AST/ASTImporterTest.cpp

   auto *ToD = Import(FromD, Lang_CXX03);
   EXPECT_TRUE(ToD);
   Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
   EXPECT_TRUE(MatchVerifier<TranslationUnitDecl>().match(
       ToTU, translationUnitDecl(hasDescendant(
                 functionDecl(hasName("f"), hasDescendant(declRefExpr()))))));
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase,
+       ImportAnnonymousUnionInClassTest) {
+  const char *Code =
+      R"(
+      class B{
+
+      public:
+
+        B(){
+          c=1;
+        }
+
+        void foo1(){}
+
+      private:
+        union{
+          int a;
+          int b;
+        };
+        union {
+          int c;
+          int d;
+        };
+      };
+      )";
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  EXPECT_TRUE(FromTU);
+}
+
+
 struct ImportFunctionTemplates : ASTImporterOptionSpecificTestBase {};
 
 TEST_P(ImportFunctionTemplates, ImportFunctionTemplateInRecordDeclTwice) {
   auto Code =
       R"(
       class X {
         template <class T>
         void f(T t);