[clangd] Filter out non-governed files from broadcast

Summary:
This also turns off implicit discovery of additional compilation
databases.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64247

llvm-svn: 365634