[mono] Allow RuntimeType arguments in CustomAttributeBuilder

[uweigand]

• Fixes [mono] CustomAttrBuilder does not support RuntimeType arguments #58019

[jkotas]

Do we need to add test?

[SamMonoRT]

@uweigand - can we get a test added for the change ?

[uweigand]

@uweigand - can we get a test added for the change ?

Turns out there was already a test case, but disabled on Mono. Re-enabling the test case showed the failure fixed by this PR originally, and it also fails because of another reason: the Mono implementation rejected a null field value of Array type, which CoreCLR accepts and the test case uses. I've added a fix for this second issue as well, and re-enabled the test case.

[ghost]

Hello @marek-safar!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[srxqds]

@marek-safar could this backport to release/6.0?

[lambdageek]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1177581011