Fix comments before labels.

Before:
switch (...) {
  // a
  // b
// c
case first:
  break;
}

After:
switch (...) {
// a
// b
// c
case first:
  break;
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179107 91177308-0d34-0410-b5e6-96231b3b80d8