[clangd] Don't make check-clangd as a dependency in check-clang-tools

Summary:
check-clang-tools will run check-clangd first, and then run the rest
tests. If clangd tests fails, check-clang-tools would be stopped.

This would block other clang-tools developers if clangd is broken.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D52781

llvm-svn: 343608