Fix ForwardCancellationTokenAnalyzer bug on default parameter values

[DavidBoike]

This fixes a bug discovered in the ForwardCancellationTokenAnalyzer and adds several more test cases.

The analyzer is supposed to ignore when you pass CancellationToken.None, but throwing an exception when doing the same thing but by using a default operator or literal which were introduced between C# 7 and 8:

// OK
await Task.Delay(100, CancellationToken.None);

// Throwing exceptions in 8.0.0
await Task.Delay(100, default(CancellationToken));
await Task.Delay(100, default);

This requires better analysis to make sure the presence of default doesn't throw it off:

public async Task SomeMethod(int a, string b, CancellationToken token = default) { }

// Would pass the 0 and null but not provide a token:
await SomeMethod(default, default);

This PR also introduces several test cases around the use of await foreach… when using async enumerables.

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Backported to release-8.0 in #6617