Add CRLF support to LineIterator.

The MRI scripts have to work with CRLF, and in general it is probably
a good idea to support this in a core utility like LineIterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221153 91177308-0d34-0410-b5e6-96231b3b80d8