[clangd] Support dexp -c "some command"

Summary:
It runs one command and exits.
See D77385 for motivation.

Reviewers: mnauw, kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77645