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

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

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79576
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 9c1b99d..27b3ae3 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1571,13 +1571,16 @@
 
   // 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())