SwitchBootstraps.ResolvedEnumLabels.resolvedEnum should be final

Current definition is:

        private final MethodHandles.Lookup lookup;
        private final EnumDesc<?>[] enumDescs;
        @Stable
        private Object[] resolvedEnum;

        public ResolvedEnumLabels(MethodHandles.Lookup lookup, EnumDesc<?>[] enumDescs) {
            this.lookup = lookup;
            this.enumDescs = enumDescs;
            this.resolvedEnum = new Object[enumDescs.length];
        }

...and `resolvedEnum` is not `final`. So technically publishing the object via data race can show `resolvedEnum` as `null`, which would break `test()` that does not expect it. Currently not a practical problem since its safety is covered by adjacent `final` fields, but should be more precise.