Allow for "value" applied to a T_ATTR to return more than one value
Add value for locexpr operations

- Not for location expression as such, just one single op.  Use @atom,
  @number, @number2 to access fields, use offset to determine offset
  inside the location expression as a whole.

- Use this to provide location expressions for DW_FORM_exprloc nodes.
  Location expressions are currently just sequences.  Dedicated type is
  upcoming.