Debugger: Fix program counter handling issue.

- For DWARF4, lowpc/highpc can be specified as constants in addition
  to addresses. Furthermore, they can also be specified such that highpc
  is a relative offset from lowpc rather than an absolute address. We
  weren't handling this case, which gcc4.8 is now using when emitting
  version 4 debug information. Fixes another part of #10659.

There still remains a problem with regards to class/structure variables not
showing up in the local variables view.
Debugger: Partly rework hrev46696.

- Various other places in Debugger expect the highpc attribute to be
  an address rather than an offset. As such, resolve it to one when
  setting the attribute in the corresponding DIE so the resolved value
  is available everywhere. Fixes variable scopes not being computed
  properly, and consequently the missing variables mentioned in the
  previous commit.

This gets things working properly again under gcc4.8.