[Foundation] Don't dispose tagged pointers. Fixes #21425.

[rolfbjarne]

Objective-C has the concept of a tagged pointer, which is not really a pointer, but instead a pointer-like value that has encoded the constant value inside itself.

More information about tagged pointers can be found online:

• https://www.mikeash.com/pyblog/friday-qa-2012-07-27-lets-build-tagged-pointers.html
• https://developer.apple.com/videos/play/wwdc2020/10163/

The important part for us is that there's no need for reference counting tagged pointers - they're constant values, and there's nothing allocated, nor does anything need to be freed either. Another point is that these tagged pointers can pop up at any time from Objective-C, and that causes a problem:

• A tagged pointer is surfaced, say for NSString, and we create a managed NSString instance around it.
• We dispose the managed NSString instance, and disconnect the managed instance from the underlying (tagged) pointer value.
• The tagged pointer pops up again, and we find the managed NSString instance in our dictionary that maps native pointers to managed instances.
• An ObjectDisposedException is thrown when using the managed NSString instance.

This PR attempts to fix this by detecting a tagged pointer, and in that case we don't disconnect the managed instance from the native pointer, because the native pointer will never point to freed memory (it'll always be a valid object).

The unfortunate part is that detecting a tagged pointer is not possible with public API, we have to look at the actual bits, and this is considered an internal implementation detail by Apple.

To make this as safe as possible:

• This feature is only enabled by default in .NET 10+.
• It's only enabled by default if the SupportedOSPlatformVersion is iOS 14+ (because earlier versions of iOS used a different tagged pointer format - see previous wwdc video link).
• It's possible to disable it by setting an MSBuild property.

TODO:

• Detect SupportedOSPlatformVersion and disable if too early.

Fixes #21425.

[filipnavara]

Isn't the default supposed to be false? How does the test pass? (Or, am I just misreading it?)

[rolfbjarne]

Yes, this was just a quick test run on CI to see if doing it this way would break any test, the PR isn't complete at all.

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