[CSGen] forcing an optional should prefer subtyping only if it's applied to a member

`TVO_PrefersSubtypeBinding` in this case is intended to support
references to Objective-C imported members that can be force
unwrapped before being used i.e. accessing an overloaded method
on `AnyObject` should always prefer an overload that returns
a subtype:

```
@objc class A: NSObject {
    @objc func test() -> A { }
    @objc func test() -> B { }
}

func test(obj: AnyObject) {
  obj.test!()
}
```

where `B` is a subtype of `A` should result in selection of
`() -> B`.

Author: xedin

lib/Sema/CSGen.cpp

@@ -2797,10 +2797,12 @@ namespace {
       // Force-unwrap an optional of type T? to produce a T.
       auto locator = CS.getConstraintLocator(expr);
 
-      auto objectTy = CS.createTypeVariable(locator,
-                                            TVO_PrefersSubtypeBinding |
-                                            TVO_CanBindToLValue |
-                                            TVO_CanBindToNoEscape);
+      auto options = TVO_CanBindToLValue | TVO_CanBindToNoEscape;
+
+      if (isExpr<UnresolvedDotExpr>(expr->getSubExpr()))
+        options |= TVO_PrefersSubtypeBinding;
+
+      auto objectTy = CS.createTypeVariable(locator, options);
 
       auto *valueExpr = expr->getSubExpr();
       // It's invalid to force unwrap `nil` literal e.g. `_ = nil!` or