LineIterator: Provide a variant that keeps blank lines

It isn't always useful to skip blank lines, as evidenced by the
somewhat awkward use of line_iterator in llvm-cov. This adds a knob to
control whether or not to skip blanks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217960 91177308-0d34-0410-b5e6-96231b3b80d8