Make Objective-C interoperability configurable in the runtime

In order to be able to debug, for example, a Linux process from a macOS host, we
need to be able to initialize a ReflectionContext without Objective-C
interoperability. This patch turns ObjCInterOp into another template trait, so
it's possible to instantiate a non-ObjC MetadataReader on a system built with
ObjC-interop (but not vice versa).

This patch changes the class hierarchy to

                          TargetMetadata<Runtime>
                                    |
                          TargetHeapMetadata<Runtime>
                                    |
                          TargetAnyClassMetadata<Runtime>
                                   /                \
                                  /               TargetAnyClassMetadataObjCInterop<Runtime>
                                 /                              \
TargetClassMetadata<Runtime, TargetAnyClassMetadata<Runtime>>    \
                                                                  \
                    TargetClassMetadata<Runtime, TargetAnyClassMetadataObjCInterop<Runtime>>

TargetAnyClassMetadataObjCInterop inherits from TargetAnyClassMetadata because
most of the implementation is the same. This choice makes TargetClassMetadata a
bit tricky. In this patch I went with templating the parent class.

rdar://87179578

Author: adrian-prantl