[clangd] Make bin/llvm-lit run standalone clangd tests

Summary:
Currently clangd lit tests can't be run in isolation because we don't
set some of the config parameters. This enables running

./bin/llvm-lit ../clang-tools-extra/clangd/test/

or any other test in that subdirectory.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73538