[clangd] Define out-of-line qualify return value

Summary:
Return type might need qualification if insertion context doesn't have
the same decls visible as the source context.

This patch adds qualification for return value to cover such cases.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70535