[Analyzer] Mark `SymbolData` parts of iterator position as live in program state maps

Marking a symbolic expression as live is non-recursive. In our checkers we
either use conjured symbols or conjured symbols plus/minus integers to
represent abstract position of iterators, so in this latter case we also
must mark the `SymbolData` part of these symbolic expressions as live to
prevent them from getting reaped.

Differential Revision: https://reviews.llvm.org/D48764

llvm-svn: 337151