[clangd] Define out-of-line availability checks

Summary:
Initial availability checks for performing define out-of-line code
action, which is a refactoring that will help users move function/method
definitions from headers to implementation files.

Proposed implementation only checks whether we have an interesting selection,
namely function name or full function definition/body.

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69266