[clangd] Implement GetEligiblePoints

Summary:
This is an helper for incoming move definition out-of-line action to
figure out possible insertion locations for definition of a qualified name.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68024