gn build: make clangd depend on clang resource headers

Summary:
clangd needs them to function properly, even though they are not
strictly required for the build.

Reviewers: thakis

Reviewed By: thakis

Subscribers: MaskRay, jkorous, arphaman, llvm-commits, kadircet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62480

llvm-svn: 361828