JSpecify: infer @Nullable type arguments for type variables from unmarked code

[msridhar]

Example:

@NullMarked
class SampleNullUnmarkedCall {

  @NullUnmarked
  static class Foo<T> {
    Foo(T t) {}

    static <U> Foo<U> id(U u) { return new Foo<>(u); }
  }

  static void test() {
    // should report no error
    Foo<@Nullable Object> x = Foo.id(null);
  }
}

Here, since Foo is @NullUnmarked, we can infer @Nullable Object as the type argument for the Foo.id(null) call, even though U (and T) do not have explicit @Nullable upper bounds.