[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'}}
}
```

include/swift/SIL/Projection.h

@@ -247,7 +247,8 @@ public:
 
   Projection &operator=(Projection &&P) = default;
 
-  bool isValid() const { return Value.isValid(); }
+  bool isValid() const { return bool(*this); }
+  operator bool() const { return Value.isValid(); }
 
   /// Convenience method for getting the underlying index. Assumes that this
   /// projection is valid. Otherwise it asserts.