Fix indentation-detection at indent level 0.

This correctly formats:
  {
    a;
  }
where { is incorrectly indented by 2, but is at level 0, when
reformatting only 'a;'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174737 91177308-0d34-0410-b5e6-96231b3b80d8