Make autocast distinct from cast

Make autocast distinct from cast

Some dwarf probes try to be helpful and fill in empty @cast modules, but
we don't want that for an automatic @cast that will determine its module
and type from its operand.  So let's just make autocast separate.

* parse.cxx (parser::parse_dwarf_value): Create autocast_op, and assign
  its token as that of the first addressof or peeked component.
* elaborate.cxx (void_statement_reducer::reduce_target_symbol): New,
  collapse any kind of target_symbol to its operand and indexes.  Call
  this in all the void_statement_reducer target_symbol-like visitors.
* [Lots of added visitor::visit_autocast_op...]