[CodeComplete] Ensure object is the same in compareOverloads()

Summary:
This fixes a regression that led to size() not being available in clangd
when completing 'deque().^' and using libc++.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68335

llvm-svn: 373710