[clangd] Propagate main context into ClangdServer

Summary:
Currently clangd initializes the ClangdServer lazily during
onInitialize request, and it results in propagation of caller's context rather
than the main context created ClangdLSPServer.

This patch changes the logic to store main context that created ClangdLSPServer
and pass it onto to ClangdServer and other objects like CDBs.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68978

llvm-svn: 374892