[gn build] Add missing clangd dependencies

Fixes
$ ninja obj/build/rel/gen/clang-tools-extra/clangd/CompletionModel.CompletionModel.obj

Some tablegen include files from clang/include/clang/AST and
clang/include/clang/Sema need to be generated before CompletionModel is
compiled.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D89657