[opt-remark] Add support for simple object projections.
This ensures that we are able to properly look through struct_extract,
tuple_extract in cases where we have an aggregate with multiple non-trivial
subtypes (implying it is not-rc identical with those sub-types).
The end result is that we now emit good diagnostics for things like this:
```
func returnStructWithOwnerOwner(x: StructWithOwner) -> Klass {
return x.owner // expected-remark {{retain}}
// expected-note @-7:33 {{of 'x.owner'}}
}
```
