Debugger: Fix LLDB reference display for LLDB 320.x

There is seemingly still no proper API for that, and
the old hack broke. Use a new one.

Change-Id: I9e638ca1fbd84c43c7d31b65e017792d1b6a1e0b
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>