[ModulePrint] Add the initial implementation for printing synthesized extensions.

For a concrete type, members from its conforming protocols' extensions can be hard
to manually surface. In this commit, when printing Swift modules, we start to replicate these
extensions and synthesize them as if they are the concrete type's native extensions.

Credit to Doug for suggesting this practice.

tools/SourceKit/lib/SwiftLang/SwiftEditorInterfaceGen.cpp

@@ -287,7 +287,7 @@ static bool getModuleInterfaceInfo(ASTContext &Ctx,
   AnnotatingPrinter Printer(Info, OS);
   printSubmoduleInterface(Mod, SplitModuleName,
                           TraversalOptions,
-                          Printer, Options);
+                          Printer, Options, false);
 
   Info.Text = OS.str();
   return false;