clang-format: Fix line breaking bug after empty ifs.

Before:
  if () {
  }
    else {
  }

After:
  if () {
  } else {
  }

This fixed llvm.org/PR17262.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190855 91177308-0d34-0410-b5e6-96231b3b80d8