[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