[clangd] Define out-of-line qualify return value
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.
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D70535