clang-format: merge short case labels with trailing comments

Summary:
Allow merging short case labels when they actually end with a comment
(like a comment after the ``break``) and when followed by switch-level
comments (e.g. aligned with next case):

  switch(a) {
  case 0: break; // comment at end of case
  case 1: return value;
  // comment related to next case
  // comment related to next case
  case 2:
  }

Reviewers: krasimir, djasper

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D35557

llvm-svn: 309370