clang-format: Make short case labels work with #ifs

Before:
  switch (a) {
  #if FOO
  case 0: return 0; #endif
  }

After:
  switch (a) {
  #if FOO
  case 0: return 0;
  #endif
  }

This fixed llvm.org/PR21544.

llvm-svn: 222642