[analyzer] Add modeling of assignment operator in smart ptr

Summary: Support for 'std::unique_ptr>::operator=' in SmartPtrModeling

Reviewers: NoQ, Szelethus, vsavchenko, xazax.hun

Reviewed By: NoQ, vsavchenko, xazax.hun

Subscribers: martong, cfe-commits
Tags: #clang

Differential Revision: https://reviews.llvm.org/D86293