[AST] Fix the PrintQualifiedName for ObjC instance variable in class extension.

[hokein]

Summary

Similar to property, we print the containing interface decl as the
nested name specifier for ivar; otherwise we will get "::ivar_name".

this would fix an assertion crash in clangd: https://github.com/clangd/clangd/issues/365

clang/lib/AST/Decl.cpp

 }
 
 void NamedDecl::printNestedNameSpecifier(raw_ostream &OS,
                                          const PrintingPolicy &P) const {
   const DeclContext *Ctx = getDeclContext();
 
   // For ObjC methods and properties, look through categories and use the
   // interface as context.
-  if (auto *MD = dyn_cast<ObjCMethodDecl>(this))
+  if (auto *MD = dyn_cast<ObjCMethodDecl>(this)) {
     if (auto *ID = MD->getClassInterface())
       Ctx = ID;
-  if (auto *PD = dyn_cast<ObjCPropertyDecl>(this)) {
+  } else if (auto *PD = dyn_cast<ObjCPropertyDecl>(this)) {
     if (auto *MD = PD->getGetterMethodDecl())
       if (auto *ID = MD->getClassInterface())
         Ctx = ID;
+  } else if (auto *ID = dyn_cast<ObjCIvarDecl>(this)) {
+    if (auto *CI = ID->getContainingInterface())
+      Ctx = CI;
   }
 
   if (Ctx->isFunctionOrMethod())
     return;
 
   using ContextsTy = SmallVector<const DeclContext *, 8>;
   ContextsTy Contexts;
 

clang/unittests/AST/NamedDeclPrinterTest.cpp

R"(
   @end
 )";
   ASSERT_TRUE(PrintedWrittenPropertyDeclObjCMatches(
     Code,
     "property",
     "Obj::property"));
 }
 
+TEST(NamedDeclPrinter, TestInstanceObjCClassExtension) {
+  const char *Code =
+R"(
+@interface ObjC
+@end
+@interface ObjC () {
+  char data; // legal with non-fragile ABI.
+}
+@end
+)";
+
+  std::vector<std::string> Args{
+      "-std=c++11", "-xobjective-c++",
+      "-fobjc-runtime=macosx" /*force to use non-fragile ABI*/};
+  ASSERT_TRUE(PrintedNamedDeclMatches(Code, Args,
+                                      /*SuppressUnwrittenScope*/ true,
+                                      namedDecl(hasName("data")).bind("id"),
+                                      // not "::data"
+                                      "ObjC::data", "input.mm"));
+}
+
 TEST(NamedDeclPrinter, TestObjCClassExtensionWithGetter) {
   const char *Code =
 R"(
   @interface Obj
   @end
 
   @interface Obj ()
   @property(nonatomic, getter=myPropertyGetter) int property;