This is because `EnablePreviewFeatures=true` doesn't quite work, since it
requires the building .NET and the target .NET to be on the same version.
We might want to build with .NET 9, but the Xcode branch is targeting .NET 8,
so it doesn't work.
This behavior is explained here:
https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md#meaning-of-property-in-multi-targeted-projects
The best solution seems to switch to using the Experimental attribute instead,
which was designed for our scenario (and explicitly to fix the problem we're
running into): bba3216250/accepted/2023/preview-apis/preview-apis.md
This also meant we had to augment `-nowarn` for bgen to:
* Pass any nowarn values to the compiler when bgen compiles stuff.
* Pass `$(NoWarn)` (the MSBuild property) to bgen when building a binding project.
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Co-authored-by: Alex Soto <alex@soto.dev>
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