Make a bit of BenchmarkDotNet trimmable

[MichalStrehovsky]

Another attempt at fixing the problem I tried to fix in #2020.

In that pull request, I reflection-rooted all of BenchmarkDotNet, but that looks like it increases the size of the closure too much (dotnet/performance#2532). So here I'm rolling that part back and annotating enough of the Characteristic APIs to make it so that we shouldn't run into the original problem.

I basically added <EnableTrimAnalyzer>true</EnableTrimAnalyzer> to the BechmarkDotNet project, looked at the trimming warning in

BenchmarkDotNet/src/BenchmarkDotNet/Characteristics/CharacteristicHelper.cs

Lines 49 to 68 in 63e28c1

	private static IReadOnlyList<Characteristic> GetThisTypeCharacteristicsCore(Type characteristicObjectType)
	{
	var fieldValues = characteristicObjectType.GetTypeInfo()
	.GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Static)
	.Where(f => IsCharacteristicSubclass(f.FieldType))
	.Select(f => AssertHasValue(f, (Characteristic)f.GetValue(null)));
	var propertyValues = characteristicObjectType.GetTypeInfo()
	.GetProperties(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Static)
	.Where(p => p.GetMethod != null && IsCharacteristicSubclass(p.PropertyType))
	.Select(p => AssertHasValue(p, (Characteristic)p.GetValue(null)));
	// DONTTOUCH: DO NOT change the order of characteristic as it may break logic of some operations.
	return fieldValues
	.Concat(propertyValues)
	.Distinct()
	.OrderBy(k => k.HasChildCharacteristics ? 1 : 0)
	.ThenBy(k => k.Id)
	.ToArray();
	}

where we were getting the crash and kept adding annotations until it stopped generating new warnings.

We now have an annotation in a spot that should make sure the Job class has all fields and properties kept (it's used with one of the annotated Create methods).

There are more trimming warnings that I didn't try to solve because they're unrelated to the problem at hand.

Cc @adamsitnik
Cc @dotnet/ilc-contrib FYI

[adamsitnik]

Looks good to me! Thanks a lot @MichalStrehovsky !