Remove unncessary whitespace when triggered on empty line.

With the cursor located at "I", clang-format would not do anything to:

int a;

   I

  int b;

With this patch, it reduces the number of empty lines as necessary, and
removes unnecessary whitespace. It does not change/reformat "int a;" or
"int b;".

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@176650 91177308-0d34-0410-b5e6-96231b3b80d8