[clangd] Make sure ReplyCallbacks are destroyed before RequestCancelersMutex

Summary:
After rL374163, replycallbacks might have a cancellable context, which
will try to access RequestCancellers on destruction. See
http://45.33.8.238/mac/1245/step_7.txt for a sample failure.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68702

llvm-svn: 374168