clang-format: Add option to allow short case labels on a single line.

On a single line:
  switch (a) {
  case 1: x = 1; return;
  case 2: x = 2; return;
  default: break;
  }

Not on a single line:
  switch (a) {
  case 1:
    x = 1;
    return;
  case 2:
    x = 2;
    return;
  default:
    break;
  }

This partly addresses llvm.org/PR16535. In the long run, we probably want to
lay these out in columns.