Classfile API cannot encode Primitive Class as Condy

Consider this code example:

    cob.constantInstruction(clazz.describeConstable().orElseThrow());

This fails for primitive classes, as Classfile API always assume ClassDescs are always encodeable as a class constant. The check in BytecodeHelpers should check for

    if (constantValue instanceof ClassDesc value && !value.isPrimitive())

As only classes or interfaces or arrays are eligible to be encoded in class constants. Discovered while converting SerialPersistentFieldsTest for JDK-8294977.