Add plugin for the Clang Language Server

Nathaniel McCallum requested to merge npmccallum/gnome-builder:clangd into master

This patch may not yet be complete. It does work. But clangd doesn't get CFLAGS or LIBS, so compilation fails pretty quickly and shows a bunch of errors in the file. I'm not super familiar with the Language Server Protocol. So I'm not sure how to provide this information. Does anyone have any suggestions?

Merge request reports