clang-format: Handle comments in short case labels.

With AllowShortCaseLabelsOnASingleLine set to true:
This gets now left unchanged:
  case 1:
    // comment
    return;

Whereas before it was changed into:
  case 1: // comment return;

This fixes llvm.org/PR21630.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222529 91177308-0d34-0410-b5e6-96231b3b80d8