[gn build] make 'clang' target depend on libcxx/include on mac

[thakis]

Summary

On macOS, libc++ headers are distributed with the compiler, not
the sysroot. Without this, compiling a file that includes something
like <string> won't compile with gn-built clang without manual tweaks.

I used to do the manual tweaks, but now that other people are starting
to use this on mac, let's make it Just Work.

(This is marginally nicer than the cmake build now in that you can
just build 'clang' and it'll do the right thing.)

llvm/utils/gn/secondary/clang/tools/driver/BUILD.gn

deps = [
     "//llvm/lib/Transforms/Utils",
     "//llvm/lib/Transforms/Vectorize",
   ]
   if (host_os == "mac") {
     deps += [ ":write_info_plist" ]
     plist = get_target_outputs(":write_info_plist")
     ldflags = [ "-Wl,-sectcreate,__TEXT,__info_plist," +
                 rebase_path(plist[0], root_out_dir) ]
+
+    # On macOS, libcxx headers are bundled with the compiler, not with the
+    # sysroot. As a convenience, automatically copy them over when buildling
+    # clang.  This is different from the CMake build, which requires devs to
+    # explicitly build the "libcxx" target (which also needlessly compiles the
+    # libcxx sources) to get a working compiler.
+    deps += [ "//libcxx/include" ]
   }
   sources = [
     "cc1_main.cpp",
     "cc1as_main.cpp",
     "cc1gen_reproducer_main.cpp",
     "driver.cpp",
   ]
 }

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

sources = [
     "utility",
     "valarray",
     "variant",
     "vector",
     "version",
     "wchar.h",
     "wctype.h",
   ]
-  deps = [ "//libcxxabi/include" ]
+  deps = []
+  if (target_os != "mac" && target_os != "win") {
+    # libcxx/cmake/Modules/HandleLibCXXABI.cmake sets
+    # LIBCXX_CXX_ABI_HEADER_TARGET if the libcxx abi library either of
+    # "libstdc++", "libsupc++", "libcxxabi", "libcxxrt", but not if it's "none",
+    # "default", or "vcruntime". So on Windows, these don't get copied due to
+    # LIBCXX_CXX_ABI_HEADER_TARGET not being set.
+    # On macOS, libcxx/CMakeLists.txt sets LIBCXX_CXX_ABI_SYSTEM to 1, which
+    # causes an empty header list to be passed to setup_abi_lib, so these
+    # don't get copied on macOS due to that.
+    deps += [ "//libcxxabi/include" ]
+  }
   if (!libcxx_needs_site_config) {
     sources += [ "__config" ]
   } else {
     deps += [ ":copy_config" ]
   }
   if (libcxx_install_support_headers) {
     sources += [
       "support/android/locale_bionic.h",