Debugger: DwarfFile: Fix DWARF4 signature handling.

Depending on how the type unit was actually set up hierarchically,
we sometimes ended up with the unit set to the wrong actual type
object. Instead, we now simply let the unit parse, and then look up
the entry by the offset specified in the header after all is said
and done. Fixes class/struct objects sometimes winding up resolving
to one of their members instead.

Some problems still remain with resolving variable values for such
types though, needs further investigation.