[analyzer] Bind return value for assigment and copies of trivial empty classes

We now properly bind return value of the trivial copy constructor
and assignments of the empty objects. Such operations do not
perform any loads from the source, however they preserve identity
of the assigned object:
```
Empty e;
auto& x = (e = Empty());
clang_analyzer_dump(x); // &e, was Unknown
```

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D155442