gn build: make clangd depend on clang resource headers

[ilya-biryukov]

Summary

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

[thakis]

Thanks for the patch!

Does the cmake build have this dependency? If so, where? I tried to find it for a bit but failed. Alternatively, if there's a reason why the gn build needs this but the cmake build doesn't, that works too :)

[ilya-biryukov]

The dependency is there in CMake, although it took me awhile to dig it out for first time.
It is added by add_clang_tool, see AddClang.cmake.

Obviously, we want the same dependency on other instances of add_clang_tool. The lack of this dependency can only be noticed when building a tool in a clean build directory without building clang. Worse yet, it will build just fine, but would fail to find the builtin headers at runtime.

[thakis]

Thanks!

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

 import("//clang-tools-extra/clangd/xpc/enable.gni")
 
 executable("clangd") {
   configs += [ "//llvm/utils/gn/build:clang_code" ]
   deps = [
     "//clang-tools-extra/clang-tidy",
     "//clang-tools-extra/clangd",
     "//clang-tools-extra/clangd:features",
     "//clang-tools-extra/clangd/refactor/tweaks",
     "//clang/lib/AST",
     "//clang/lib/Basic",
     "//clang/lib/Format",
     "//clang/lib/Frontend",
+    "//clang/lib/Headers",
     "//clang/lib/Sema",
     "//clang/lib/Tooling",
     "//clang/lib/Tooling/Core",
     "//llvm/lib/Support",
   ]
   if (clangd_build_xpc) {
     deps += [
       "//clang-tools-extra/clangd/xpc:conversions",