fix viewpoint adaptation

[AndrewShf]

According to the manual (https://eisop.github.io/cf/manual/manual.html#type-variable-use), if the type variable use and the corresponding type argument both are annotated by type qualifiers, we will keep the one presented on the type variable use.
For example,

class MyClass<T> {
  ...
  @Nullable T myField = null;
  ...
}

The myField field of the local variable MyClass<@NonNull Object> mc still has type @Nullable.

However, this rule is broken in context-sensitive type systems (For example, PICO, viewpointtest)

Consider the viewpointtest checker

@SuppressWarnings("cast.unsafe.constructor.invocation")
class Demo<E> {
    @A E element;

    static void test() {
        @A Demo<@B Element> d = new @A Demo<@B Element>();
        // d.element = @A |> @ A = @A
        // thus expects no error here
        d.element = new @A Element();
    }
}

class Element {
    int f = 1;
}

viewpointtest is a context-sensitive type system for testing purposes. @A, @B are not incompatible type qualifiers. It acts differently than what we have discussed earlier when doing type substitution for d.element, replacing @A E with @B Element, instead of keeping @A on the field, and would report an assignment.type.incompatible error in the assignment of the static test method.

We realize it is in the process of viewpoint adaptation, the type substitution goes wrong, and we fix the bug there.

[wmdietl]

Closed in favor of #799.