Preserve trailing commas for enums with members when trailing_commas are preserved

[jellynoone]

This PR changes the formatter to have it maintain trailing commas of last enum constants when there are members.

So:

enum E { a, b, c,; int x; }

Becomes:

enum E {
  a,
  b,
  c,
  ;

  int x;
}

Instead of:

enum E {
  a,
  b,
  c;

  int x;
}

Fixes #1678

Additionally, a similar style choice was presented in #1660 (comment)

Affected users:

• Those passing the trailing_commas: preserve config option and adding a trailing comma to their enum cases.
• Users who don't already pass a trailing comma, aren't affected. So this PR shouldn't have undesirable effects on existing users.

Benefits:

• Maintains developer's intent
• Less churn when adding additional member cases
  (Personally, I always add a trailing semicolon to avoid churn and to also make it even clearer this is an advanced enum with members.)

Downsides:

• 7 additional logical lines of code to maintain

Additional considerations:
I also considered implementing this for enum E { a, b,; }, however, that would require more drastic changes to the formatter as far as I can tell.

Additionally, in this case the trailing semicolon provides fewer benefits:

• churn when adding new members is the same if there was a trailing semicolon or not
• giving the enum other members also only amounts to new lines being added rather than changed so again, no real churn.

Notes

• One test was changed due to this being a change in behavior.
• This was surprisingly easy and enjoyable to implement (compliment to the existing code base).
• I didn't make any changes to the CHANGELOG.md since this trailing commas preservation hasn't shipped yet so its behaviour is still undefined.

[jellynoone]

@munificent I saw your recent changes guarding the formatter with a language version flag. Please advise if the changes here require any additional modifications.

FYI, I pulled in the new changes, and all ran successfully.