Debugger: Use SBTarget.CreateValueFromAddress instead of SBValue

SBTarget.CreateValueFromAddress was not supported by older
versions of LLDB. Unfortunately, we still can't use
SBTarget.CreateValueFromExpression.

Change-Id: I982021531a2ab070aa279dd17e9714f86efe83cc
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>