[clangd] Add "check-clangd" target

Summary:
So we don't have to run "check-clang-tools" (which builds and tests all
clang tools) to verify our clangd-related change. It'd save waiting time for
clangd developers.

check-clangd (build ~1000 files, run ~340 tests) vs check-clang-tools (build
~3000 files, run ~1000 tests).

In the future, we probably want to add similar target for other
clang-tools (e.g. clang-tidy).

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D52710

llvm-svn: 343474