Improve doctor message when xcode-select invocation required (#8179)

If a developer has installed the Xcode command-line tools, then later
installs Xcode, they may need to run xcode-select to update the path of
the command-line tools to that of the new installation.

packages/flutter_tools/lib/src/ios/ios_workflow.dart

@@ -96,6 +96,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
         messages.add(new ValidationMessage.error(
             'Xcode installation is incomplete; a full installation is necessary for iOS development.\n'
             'Download at https://developer.apple.com/xcode/download/.'
+            'Once installed, run \'sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer\'.'
         ));
       }
     }