DWARF says line number 0 is a valid line number - used to indicate a source line that should 
not have breakpoints set on it inserted into code that does have a valid line number.  So allow
that line number, and the ThreadPlanStepRange should just continue stepping over 0 line ranges
as if they had the same line number as whatever we were previously stepping through.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191477 91177308-0d34-0410-b5e6-96231b3b80d8