Add support for weak_ptr in C++11?

[bernardnormier]

We should consider adding a metadata "cpp:weak" that remaps shared_ptr to weak_ptr to let the user avoid strong cycles. This metadata could apply to:

• operation parameters of type class
• struct and class data members of type class (I don't see how a cycle could be possible with an exception)
• sequence<V> and dictionaries<K, V>, where V is a class

ICE-8546