[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.

Author: nkcsgexi

include/swift/AST/PrintOptions.h

@@ -20,6 +20,7 @@ namespace swift {
 class GenericParamList;
 class CanType;
 class ExtensionDecl;
+class NominalTypeDecl;
 class TypeBase;
 class DeclContext;
 class Type;
@@ -205,6 +206,8 @@ struct PrintOptions {
 
   std::shared_ptr<PrinterArchetypeTransformer> pTransformer;
 
+  NominalTypeDecl *SynthesizedTarget = nullptr;
+
   /// Retrieve the set of options for verbose printing to users.
   static PrintOptions printVerbose() {
     PrintOptions result;
@@ -247,8 +250,13 @@ struct PrintOptions {
   }
 
   static PrintOptions printTypeInterface(Type T, const DeclContext *DC);
+
   void setArchetypeTransform(Type T, const DeclContext *DC);
 
+  void initArchetypeTransformerForSynthesizedExtensions(NominalTypeDecl *D);
+
+  void clearArchetypeTransformerForSynthesizedExtensions();
+
   /// Retrieve the print options that are suitable to print the testable interface.
   static PrintOptions printTestableInterface() {
     PrintOptions result = printInterface();

include/swift/IDE/ModuleInterfacePrinting.h

@@ -40,13 +40,15 @@ typedef OptionSet<ModuleTraversal> ModuleTraversalOptions;
 
 void printModuleInterface(ModuleDecl *M,
                           ModuleTraversalOptions TraversalOptions,
-                          ASTPrinter &Printer, const PrintOptions &Options);
+                          ASTPrinter &Printer, const PrintOptions &Options,
+                          const bool PrintSynthesizedExtensions);
 
 // FIXME: this API should go away when Swift can represent Clang submodules as
 // 'swift::Module *' objects.
 void printSubmoduleInterface(ModuleDecl *M, ArrayRef<StringRef> FullModuleName,
                              ModuleTraversalOptions TraversalOptions,
-                             ASTPrinter &Printer, const PrintOptions &Options);
+                             ASTPrinter &Printer, const PrintOptions &Options,
+                             const bool PrintSynthesizedExtensions);
 
 /// Print the interface for a header that has been imported via the implicit
 /// objc header importing feature.