Support nint/intptr conversion on c# 11 and up

[CyrusNajmabadi]

Fixes #74973

[ToddGrun]

For my benefit, why does this use the markup/workspace string format rather than just specifying the string as most of the tests do in this file?

[CyrusNajmabadi]

because it needs to specify the set of references/runtime the project is configured against. For the legacy test framework (the one that is not using the VerifyCS pattern) this can only be done with this <Workspace> format. For the modern test framework, you can do ReferenceAssemblies = ReferencesAssemblies.Net.Net80 and stuff like that.

[ToddGrun]

[tannergooding]

@CyrusNajmabadi, I don't believe this is correct.

The C# 9 feature was to introduce nint as a language keyword, but the behavior of nint and IntPtr could differ and as such they are not considered equivalent.

In C# 11, it was changed so that if the runtime feature exists then the language can assume nint and IntPtr have identical behavior and that it is safe to treat them as strict aliases of eachother.

Accordingly, even if a user manually targets C# 11 then on a runtime (such as .NET Framework) without the corresponding runtime feature define, then nint.M() and IntPtr.M() can differ in behavior and this can lead to quirks and other bugs.

[tannergooding]

This is covered in the feature speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/numeric-intptr.md

Which explicitly states that the new behavior is only triggered by existence of the System.Runtime.CompilerServices.RuntimeFeature.NumericIntPt runtime feature flag.

[CyrusNajmabadi]

Will fix in followup. Thanks!