C++: Improve Iterator models

[geoffw0]

Improve Iterator.qll models:

• for the most part Iterator.qll already models its classes, and uses a special predicate getIteratorArgumentInput to connect classes to non-member function models. But there were a few opportunities to clean things up for member function models using getClassAndName.
• there were some issues with the operator+= models, particularly with the non-member version and consistency between the member and non-member versions; I've added some more test cases and attempted to improve matters. But please do say if you think I may have made a mistake!

Regarding BSL:

• the bsl classes appear to have iterators, begin methods etc with the normal behaviour.
• The bsl::iterator, bsl::front_inserter, bsl::inserter, and bsl::back_inserter functions don't appear to be documented but are implemented as references (via using declarations) to the std versions.
• depending on macros there is a bsl:iterator_traits corresponding to std::iterator_traits, or a using. I couldn't find documentation for it but it's clearly intended to be the same thing.
• there are also a handful of special iterators (I think), such as bsl::List_Iterator. This example should be caught by IteratorByTypedefs and modelled correctly.

TL;DR: bsl iterators work the same as std iterators, so I've changed the model to include them, and also fixed a few things.

[criemen]

LGTM, but somebody else should double-check the new taint flow.

[jbj]

somebody else should double-check the new taint flow.

@rdmarsh2, can you check it?

[criemen]

As Robert didn't complain about the new taint flow, and his comment has been addressed, LGTM.