[clangd] Wait for compile command in ASTWorker instead of ClangdServer
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
Differential Revision: https://reviews.llvm.org/D60607