Format a line if a range in its leading whitespace was selected.

With [] marking the selected range, clang-format invoked on

    [  ]   int a;

Would so far not reformat anything. With this patch, it formats a
line if its leading whitespace is touched.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176435 91177308-0d34-0410-b5e6-96231b3b80d8