[registrar] Fix registration of stub Objective-C classes. Fixes #21546.

[rolfbjarne]

Objective-C has a fairly new feature called stub (unrealized) classes, which don't exist until runtime. They were introduced to support Swift classes exposed to Objective-C, when the exact layout of the class isn't known at compile time, and the creation of the class itself is deferred until runtime.

An Objective-C class is marked as a stub class by adding the objc_class_stub attribute to the class definition:

__attribute__((objc_class_stub)
@interface ObjCStubClass : NSObject
@end

Sidenote: the class will also have the objc_subclassing_restricted attribute, because Clang requires the latter when the former is present.

When this attribute is present, Clang will generate code to load the class at runtime, instead of just fetching it, when doing [ObjCStubClass class].

Our immediate symptom of stub classes when using the static registrar is that the above would crash:

• We don't have the original headers for any third-party Objective-C code, so we generate it based on the information we have available.
• We don't add the objc_class_stub attribute to the generated Objective-C header (because we don't know that the class is a stub class).
• Clang compiles the wrong code for [ObjCStubClass class], because of the missing attribute.

The first step in the fix is to add a new IsStubClass property to the Register attribute, so that we can let the registrars know about them.

Then update the static registrar to add the objc_class_stub attribute to the generated Objective-C header.

We also need a fix for the dynamic registrar, because we call the objc_getClass ("ObjCStubClass") there to get the Objective-C class, and that doesn't work unless the stub class has already been realized/used (objc_getClass returns NULL). It turns out that computing the total number of Objective-C classes in the process will also realize all stub Objective-C classes, so that's what we do when we find a stub Objective-C class.

References:

• https://github.com/swiftlang/swift/blob/main/docs/ObjCInterop.md#stub-classes
• https://alwaysprocessing.blog/2023/02/16/objc-unrealized-classes

Fixes #21546.

[vs-mobiletools-engineering-service2]

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[rolfbjarne]

/azp run xamarin-macios-pr

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[rolfbjarne]

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[rolfbjarne]

Test failure is unrelated (#21864).