[clangd] Reland Store Index in Tweak::Selection
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.
Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Differential Revision: https://reviews.llvm.org/D69165