[clangd] Add CMake dependencies for Protobuf-generated files

Summary:
Dependencies ensure that Protobufs are generated before all libraries
depending on the headers are **built**, not linked.

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79085