Consider targets of delegates reflection-visible

[MichalStrehovsky]

This is to make sure Delegate.GetMethodInfo API works in IlcTrimMetadata=true mode. In this mode, the presence of code doesn't automatically mean the method is visible to reflection (we only consider results of dataflow analysis or XML inputs, or DynamicDependency). Add delegate targets to this list so that GetMethodInfo API reliably works.

The compiler already tracks delegate creation sequence, so adding a callback to MetadataManager to inject the dependencies that make the method reflection-visible.

We also handle the situation when the delegate was created to a virtual method and the exact target isn't known until runtime. We do this by injecting conditional dependencies on virtual method implementations.

This change causes a 0.3% size regression on ASP.NET WebApi template with IlcTrimMetadata=true. I spot checked the diffs and they all look correct (there's a lot of delegates being created to support various captures and suddenly those things become reflectable - it's what we want).

[jkotas]

Thanks!

[MichalStrehovsky]

Cc @dotnet/ilc-contrib

This is implementing a thing that IL Linker currently doesn't do:

Action<string> a1 = new Base().Method;
Console.WriteLine(a1.Method.GetParameters()[0].Name);

Action<string> a2 = new Derived().Method;
Console.WriteLine(a2.Method.GetParameters()[0].Name);

class Base
{
    public virtual void Method(string arg) { }
}

class Derived : Base
{
    public override void Method(string arg) { }
}

Will produce a different output after trimming with IL Linker because we stripped the parameter names. The behavioral difference with NativeAOT would be more catastrophic because we strip more than parameter names and implementing this was more urgent.