[change-namespace] get insertion points of forward declarations correct.

[ioeric]

Summary

Previously, the insertion points would conflict with the old namespace
deletion.

[hokein]

unittests/change-namespace/ChangeNamespaceTests.cpp

354

I'd create a new test for the fix instead of modifying the current one.

[ioeric]

• Create a new test case. And slight refactor.

[hokein]

LGTM.

change-namespace/ChangeNamespace.cpp

void ChangeNamespaceTool::moveClassForwardDeclaration(
   // Insert the forward declaration back into the old namespace after moving the
   // code from old namespace to new namespace.
   // Insertion information is stored in `InsertFwdDecls` and actual
   // insertion will be performed in `onEndOfTranslationUnit`.
   // Get the (old) namespace that contains the forward declaration.
   const auto *NsDecl = Result.Nodes.getNodeAs<NamespaceDecl>("ns_decl");
   // The namespace contains the forward declaration, so it must not be empty.
   assert(!NsDecl->decls_empty());
-  const auto Insertion = createInsertion(NsDecl->decls_begin()->getLocStart(),
-                                         Code, *Result.SourceManager);
+  const auto Insertion =
+      createInsertion(getLocAfterNamespaceLBrace(NsDecl, *Result.SourceManager,
+                                                 Result.Context->getLangOpts()),
   InsertForwardDeclaration InsertFwd;
   InsertFwd.InsertionOffset = Insertion.getOffset();
   InsertFwd.ForwardDeclText = Insertion.getReplacementText().str();
   InsertFwdDecls[Insertion.getFilePath()].push_back(InsertFwd);
 }
 
 // Replaces a qualified symbol (in \p DeclCtx) that refers to a declaration \p
 // FromDecl with the shortest qualified name possible when the reference is in

unittests/change-namespace/ChangeNamespaceTests.cpp

std::string Expected = "namespace na {\n"
                          "} // namespace y\n"
                          "} // namespace x\n";
   EXPECT_EQ(format(Expected), runChangeNamespaceOnCode(Code));
 }
 
 TEST_F(ChangeNamespaceTest, LeaveForwardDeclarationBehind) {
   std::string Code = "namespace na {\n"
                      "namespace nb {\n"
+                     "\n"
                      "class FWD;\n"
                      "class FWD2;\n"
                      "class A {\n"
                      "  FWD *fwd;\n"
                      "};\n"
+                     "\n"
                      "} // namespace nb\n"
                      "} // namespace na\n";
   std::string Expected = "namespace na {\n"
                          "namespace nb {\n"
                          "class FWD;\n"
                          "class FWD2;\n"
                          "} // namespace nb\n"
                          "} // namespace na\n"
                          "namespace x {\n"
                          "namespace y {\n"
                          "\n"
                          "class A {\n"
                          "  ::na::nb::FWD *fwd;\n"
                          "};\n"
+                         "\n"
                          "} // namespace y\n"
                          "} // namespace x\n";
   EXPECT_EQ(format(Expected), runChangeNamespaceOnCode(Code));
 }
 
 TEST_F(ChangeNamespaceTest, TemplateClassForwardDeclaration) {
   std::string Code = "namespace na {\n"
                      "namespace nb {\n"