Fix how COM objects are enumerated

[daxian-dbw]

PR Summary

Fix #11782

The ComEnumerator was introduced back in early .NET Core 2.0 preview period of time (#4553), because GetEnumerator() didn't work on COM object even if the object can be cast to IEnumerable and .NET Core team said it was by design at the time (dotnet/runtime#21690).
With .NET Core 3.1, GetEnumerator() works on the COM objects that can be cast to IEnumerable 🎉
So we can directly use the .NET Core support to get the enumerator for a COM object that implements IEnumerable.

However, for the COM object that can be cast to IEnumerator, exception is thrown when calling MoveNext() on it.
So we still need the ComEnumerator, but much simplified to just cover the case that the COM object implements COM.IEnumVARIANT interface.

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[SteveL-MSFT]

Are there any specific names we can validate against that are always expected for this COM object?

[daxian-dbw]

I guess 'Authenticated Users' and 'INTERACTIVE'. Let me update and test it out.

[daxian-dbw]

Test updated.

[PaulHigin]

LGTM

[adityapatwardhan]

@SteveL-MSFT please update your review

[ghost]

🎉v7.0.0-rc.3 has been released which incorporates this pull request.:tada:

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