[clangd] Fix crash if pending computations were active on exit

Summary:
Make sure JSONRPCDispatcher outlives the worker threads, they access
its fields to remove the stored cancellations when Context dies.

Reviewers: sammccall, ioeric

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52420

llvm-svn: 343067