AST: Tweak a DynamicSelfType hack in the ASTPrinter

Let's use transformRec() instead of subst() here since we want to
leave DependentMemberTypes unchanged. This avoids an assertion
failure with the upcoming change to InFlightSubstitution::lookupConformance().

Author: slavapestov

lib/AST/ASTPrinter.cpp

@@ -4334,13 +4334,14 @@ void PrintAST::visitFuncDecl(FuncDecl *decl) {
       if (proto && Options.TransformContext) {
         auto BaseType = Options.TransformContext->getBaseType();
         if (BaseType->getClassOrBoundGenericClass()) {
-          ResultTy = ResultTy.subst(
-            [&](Type t) -> Type {
-              if (t->isEqual(proto->getSelfInterfaceType()))
+          ResultTy = ResultTy.transformRec(
+            [&](TypeBase *t) -> std::optional<Type> {
+              if (isa<DependentMemberType>(t))
+                return t;
+              else if (t->isEqual(proto->getSelfInterfaceType()))
                 return DynamicSelfType::get(t, Ctx);
-              return t;
-            },
-            MakeAbstractConformanceForGenericType());
+              return std::nullopt;
+            });
           ResultTyLoc = TypeLoc::withoutLoc(ResultTy);
         }
       }