[Refactor Xcode Objects] Enable generating per file '--help' compiler flag

tools/gn/xcode_writer.cc

Index: tools/gn/xcode_writer.cc
diff --git a/tools/gn/xcode_writer.cc b/tools/gn/xcode_writer.cc
index 633a1c8db5ea35b354ff3bfb5297aa3c1628dae4..02ef542889fed43e65ba839641e86c50cbcb6225 100644
--- a/tools/gn/xcode_writer.cc
+++ b/tools/gn/xcode_writer.cc
@@ -353,7 +353,8 @@ void XcodeWriter::CreateSourcesProject(
   for (const SourceFile& source : sources) {
     std::string source_file =
         RebasePath(source.value(), source_dir, absolute_source_path);
-    sources_for_indexing->AddSourceFile(source_file);
+    sources_for_indexing->AddSourceFile(

[sdefresne, 2016/12/17 00:07:27]

Please add a comment here explaining why you add the --help flag to the compiler
command-line. I know why (this is explained in your document), but future reader
of this code may wonder why. Could be as simple as saying that the file need to
be know to Xcode for proper indexing and for discovery of tests function for
XCTest but that the compilation is done via ninja and thus must be disabled in
Xcode via this hack.

[liaoyuke, 2016/12/19 08:02:40]

Ack. Here we are only adding files to "sources", but will keep in mind to add
the comment later.

+        source_file, PBXBuildFile::HelpCompilerFlagEnabled::HELP_FLAG_DISABLED);
   }
 
   projects_.push_back(std::move(sources_for_indexing));