Class::reflectionData was changed from a SoftReference to a direct reference in the leyden/premain branch, as the AOT cache didn't support Reference objects at the time

https://github.com/openjdk/leyden/blame/432069bf72af6c26a922a0fe9a6b20b06c7c0599/src/java.base/share/classes/java/lang/Class.java#L2848

- private transient volatile SoftReference<ReflectionData<T>> reflectionData;
+ private transient volatile ReflectionData<T> reflectionData;

Since JDK-8341587, we can store Reference objects into the AOT cache. Therefore, we should revert the above change.

Because the reflectionData object is stored in a SoftReference, there's a chance that it may be garbage collected in the assembly phase. To guard against this, after each call to AOTConstantPoolResolver::generate_reflection_data(), we should add the generated reflectionData into a global root to make sire it's not collected.