Verify cancellation behavior of `EnumValidationGenerator`

[Youssef1313]

winforms/src/System.Windows.Forms.PrivateSourceGenerators/src/System/Windows/Forms/SourceGenerators/EnumValidationGenerator.cs

Lines 211 to 214 in f2060df

	if (cancellationToken.IsCancellationRequested)
	{
	yield break;
	}

This feels suspicious. I think it may happen that the compiler caches this step, causing incomplete result to be cached and re-used. This should use ThrowIfCancellationRequested.

(Putting aside that the generator is likely to have severe perf issues)