Avoid CollectionsMarshal.AsSpan for derived List<T>

[xtqqczze]

• Task.WaitAll
• Task.Whenall
• string.Join

As per:

runtime/src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

Line 925 in a693af3

if (values.GetType() == typeof(List<string?>)) // avoid accidentally bypassing a derived type's reimplementation of IEnumerable<T>

Related: #118682

[stephentoub]

I appreciate what you're trying to do here, but I'm really not motivated to make these changes. Can you help me understand why you believe this is important? If you derive from List<T>, you can't override any of its meaningful methods; anyone who's changing behavior in a derived type's IEnumerable<T> implementation can't have any expectations about how a consumer uses that instance and which behaviors will be employed, as it will change based on how the instance is consumed.

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[jeffhandley]

@xtqqczze As with #118682, I'm closing this without merge. If there's a challenge introduced with the current behavior that you think should be addressed, please file an issue where we can discuss the trade-offs.

Generally speaking, behavioral changes need to be agreed upon in an issue before a PR. Thank you for the contributions though.