ClassTemplateDecls are duplicated when parser already created a definition and a fwd decl

This test is failing:

TEST_P(ASTImporterTestBase,
       ImportDefinitionOfClassTemplateIfThereIsAnExistingFwdDeclAndDefinition) {
  Decl *ToTu = getToTuDecl(
      R"(
      template <typename T>
      struct B {
        void f();
      };

      template <typename T>
      struct B;
      )",
      Lang_CXX);

  Decl *FromTU = getTuDecl(
      R"(
      template <typename T>
      struct B {
        void f();
      };
      )",
      Lang_CXX, "input1.cc");
  ClassTemplateDecl *FromD = FirstDeclMatcher<ClassTemplateDecl>().match(
      FromTU, classTemplateDecl(hasName("B")));
  Import(FromD, Lang_CXX);
  // We should have only one definition.
  EXPECT_EQ(1u, DeclCounterWithPredicate<ClassTemplateDecl>(
                    [](const ClassTemplateDecl *T) {
                      return T->isThisDeclarationADefinition();
                    })
                    .match(ToTu, classTemplateDecl()));
}

Ericsson / clang

ClassTemplateDecls are duplicated when parser already created a definition and a fwd decl #385