V4 Debugger: Avoid special refs when looking up normal ones

V4 Debugger: Avoid special refs when looking up normal ones

The "values" of "special" refs are always JavaScript "undefined"s.
Thus, when trying to deduplicate an actual "undefined" we'd
accidentally hit the special refs. Avoid this by checking if a found
ref is special.

In an ideal world we'd get rid of the special refs altogether and
save QV4::FunctionObject in the values array, but that is not quite
trivial as long as the QV4::ExecutionEngine::stackTrace() doesn't
give us FunctionObjects.

Task-number: QTBUG-47788
Change-Id: Idf358c285f40930220fad2207c6ab5c9101573b2
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>