chore(tslint): custom rules invoke superclass methods (#6215)

A few rules are not implemented properly because their associated walker is overwriting hooks from the `SyntaxWalker` superclass. Those custom hooks need to invoke the method on the superclass as well.

See https://palantir.github.io/tslint/develop/custom-rules/

tools/tslint-rules/noExposedTodoRule.js

@@ -28,7 +28,9 @@ class NoExposedTodoWalker extends Lint.RuleWalker {
         this.addFailureAt(commentRange.pos, commentRange.end - commentRange.pos, ERROR_MESSAGE);
       }
     });
+
+    super.visitSourceFile(sourceFile);
   }
 }
 
-exports.Rule = Rule;
+exports.Rule = Rule;

tools/tslint-rules/requireLicenseBannerRule.js

@@ -50,6 +50,8 @@ class RequireLicenseBannerWalker extends Lint.RuleWalker {
     if (licenseCommentPos !== 0) {
       return this.addFailureAt(0, 0, ERROR_MESSAGE, tslintFix);
     }
+
+    super.visitSourceFile(sourceFile);
   }
 }