-Ystatistics accepts a list of phases for which to print stats

Also cleaned up handling of defaults in MultiChoiceSetting.
I see that I started with the following test (which makes no sense), then ultimately consulted javac.
```
+ // two lint settings are first come etc; unlike -Y
+ assertTrue(check("-Xlint", "-Xlint:-warn-missing-interpolator")(_.warnMissingInterpolator))
```