Handle enhanced for loops properly in JSpecify mode

Example:
```java
void test(List<@Nullable String> l) {
  for (String s: l) {
    s.hashCode(); // should report a warning
  }
  for (var s: l) {
    s.hashCode(); // should report a warning
  }
}
interface Foo<T extends @Nullable Object> { T get(); }
void test2(List<Foo<@Nullable String>> l) {
  for (var foo: l) {
    foo.get().hashCode(); // should report a warning
  }
}
```