[clangd] Introduced a Context that stores implicit data

Summary:
It will be used to pass around things like Logger and Tracer throughout
clangd classes.

Reviewers: sammccall, ioeric, hokein, bkramer

Reviewed By: sammccall

Subscribers: klimek, bkramer, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D40485

llvm-svn: 320468