[clangd] Store Index in Tweak::Selection

Summary:
Incoming define out-of-line tweak requires access to index.

This patch simply propogates the index in ClangdServer to Tweak::Selection while
passing the AST. Also updates TweakTest to accommodate this change.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69165