[clangd] Wait for compile command in ASTWorker instead of ClangdServer

Summary:
This makes addDocument non-blocking and would also allow code completion
(in fallback mode) to run when worker waits for the compile command.

Reviewers: sammccall, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: javed.absar, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60607

llvm-svn: 358400