Refine the nullability logic for type variables.

[copybara-service]

Refine the nullability logic for type variables.

We allow an AutoValue or AutoBuilder property of type T to be null (meaning we omit null checks) if T has a nullable bound, for example <T extends @Nullable Object>. But previously we implemented this in a confusing way, essentially pretending that T had its own @Nullable annotation. The problem is that sometimes that would lead us not to add @Nullable to a declaration of type T because we thought there already was one. For example, non-primitive builder fields start out null, so they should be declared @Nullable even if the final built property is not @Nullable. That didn't work for <T extends @Nullable Object>: because we were pretending that T already had a @Nullable annotation, we ended up declaring the builder field as private T bar; instead of private @Nullable T bar;.

To fix this problem, we stop pretending that <T extends @Nullable Object> has a @Nullable annotation, and instead introduce a separate method that specifically tests whether T is a type variable with a @Nullable bound.

RELNOTES=A problem has been fixed where builder field declarations for properties whose type was a type variable with a @Nullable bound would not have their own @Nullable marker.