CPP: Use newer taint tracking in Overflowdest.ql

[geoffw0]

Switch Overflowdest.ql from security.TaintTracking to newer dataflow.TaintTracking as has been requested. See discussion in #329. This will cause some test failures and should not be merged until the latter taint library has been extended to handle cases such as:

char array[128];

taintSource(array);

// ...

taintSink(array);

(@rdmarsh2 estimates early November)

In addition we should probably run query differences of some sort on this query once the tests pass. And probably give it a change note.

[jbj]

I've added the WIP label to make sure this isn't accidentally merged.

[geoffw0]

@rdmarsh2 do we still have plans to implement the above described improvement to dataflow.TaintTracking? Or should we skip this and wait for IR-based taint tracking? Either way, without this feature I think we'll struggle to move away from the old security.TaintTracking.

[jbj]

Plans have changed: we'll switch all the security.TaintTracking users over to IR taint tracking at the same time via DefaultTaintTracking.qll.

[geoffw0]

OK.