[gn build] Add missing clangd dependencies

[aeubanks]

Summary

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.

[aeubanks]

Not really sure what the proper way to depend on indirectly included headers is...

[thakis]

Oh sorry didn't see the question about the proper way.

The Right Way to do this in theory: gn check can be used to verify that a target only includes headers from targets in the deps list. You can then put all public headers of a directory in a target and make gn check that all deps are correct. If a target's public headers include generated headers, the public header target would then have a dep on the generation action and everything works well.

However, llvm's cmake files don't list .h files in the cmakelist files, and I don't want to keep .h lists updated in the gn build without the cmake ground truth. (Actually, maybe sync_from_cmake.py could sync .h files against disk globs instead?). If we did list .h files, there'd probably be targets for {clang,llvm,...}/include/Foo and every lib target would depend on the header targets it included, or something. This is the "investigate feasibility of gn check" in llvm/utils/gn/TODO.txt. At the moment, the llvm gn bots always build everything and run all tests if any .h file is touched since gn doesn't know about .h files. If it did, https://github.com/nico/hack/blob/master/llvmgnsyncbot/syncbot.py#L143 could not list .h and then a change touching a .h file in clang would only run clang and clang-tidy and clangd and compiler-rt tests, not llvm tests. So that'd be lowkey nice.

But for now, I think this here as-is is an ok-if-a-bit-iffy fix.

llvm/utils/gn/secondary/clang-tools-extra/clangd/BUILD.gn

write_cmake_config("features") {
   values += [ "CLANGD_ENABLE_REMOTE=0" ]
   public_configs = [ ":features_config" ]
 }
 
 gen_decision_forest("CompletionModel") {
   model = "quality/model"
   filename = "CompletionModel"
   cpp_class = "clang::clangd::Example"
+  deps = [ "//clang/lib/Sema" ]
 }
 
 static_library("clangd") {
   output_name = "clangDaemon"
   configs += [ "//llvm/utils/gn/build:clang_code" ]
   deps = [
     ":CompletionModel",
     ":features",

llvm/utils/gn/secondary/clang-tools-extra/clangd/quality/gen_decision_forest.gni

 template("gen_decision_forest") {
   model = invoker.model
   filename = invoker.filename
   cpp_class = invoker.cpp_class
+  if (defined(invoker.deps)) {
+    extra_deps = invoker.deps
+  } else {
+    extra_deps = []
+  }
 
   action("${target_name}Cpp") {
     script = "//clang-tools-extra/clangd/quality/CompletionModelCodegen.py"
     outputs = [
       "$target_gen_dir/$filename.cpp",
       "$target_gen_dir/$filename.h",
     ]
     inputs = [
config("${target_name}Config") {
     cflags = [ "-Wno-unused-label" ]
   }
 
   source_set(target_name) {
     configs += [
       "//llvm/utils/gn/build:clang_code",
       ":${target_name}Config",
     ]
-    deps = [ ":${target_name}Cpp" ]
+    deps = [ ":${target_name}Cpp" ] + extra_deps
     include_dirs = [ "." ]
     sources = get_target_outputs(":${target_name}Cpp")
   }
 }