Add explicit return undefined;

(cherry picked from commit 1f503f1b27)

src/compiler/checker.ts

@@ -10359,6 +10359,8 @@ namespace ts {
             if (returnTag && returnTag.typeExpression) {
                 return getTypeFromTypeNode(returnTag.typeExpression.type);
             }
+            
+            return undefined;
         }
 
         function createPromiseType(promisedType: Type): Type {