Demangler: add an option to print a demangled type name exactly as the runtime function _typeName would do.

Author: eeckstein

include/swift/Demangling/Demangle.h

@@ -62,6 +62,8 @@ struct DemangleOptions {
   bool DisplayDebuggerGeneratedModule = true;
   bool DisplayStdlibModule = true;
   bool DisplayObjCModule = true;
+  bool PrintForTypeName = false;
+
   /// If this is nonempty, entities in this module name will not be qualified.
   llvm::StringRef HidingCurrentModule;
   /// A function to render generic parameter names.

lib/Demangling/NodePrinter.cpp

@@ -1131,8 +1131,12 @@ NodePointer NodePrinter::print(NodePointer Node, bool asPrefixContext) {
       Printer << "):";
     }
     print(Node->getChild(1));
-    if (Node->getNumChildren() == 3)
-      print(Node->getChild(2));
+    if (Node->getNumChildren() == 3) {
+      // Currently the runtime does not mangle the generic signature.
+      // This is an open to-do in swift::_buildDemanglingForContext().
+      if (!Options.PrintForTypeName)
+        print(Node->getChild(2));
+    }
     return nullptr;
   case Node::Kind::Variable:
     return printEntity(Node, asPrefixContext, TypePrinting::WithColon,