[lldb/ObjC] Make the NonPointerIsaCache initialization lazy

The objc_debug_isa_class_mask magic value that the objc runtime vends
is now initialized using a static initializer instead of a constant
value. The runtime plugin itself will be initialized before the value
is computed and as a result, the cache will get the wrong value.

Making the creation of the NonPointerIsaCache fully lazy fixes this.