[clangd] abort if shutdown takes more than a minute.

Summary:
A certain class of bug (e.g. infloop on an AST worker thread) currently means
clangd never terminates, even if the editor shuts down the protocol and closes
our stdin, and the main thread recognizes that.

Instead, let's wait 60 seconds for threads to finish cleanly, and then crash
if they haven't.

(Obviously, we should still fix these bugs).

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69329