EnC: Allow adding/removing/updating attributes

[tmat]

Allow all custom attributes on .NET Core.
Allow specific (nullability) custom attributes on .NET Framework.

• Debugger needs to trigger an event that notifies Reflection to invalidate caches in the debuggee - via func-eval.
  The event has well-known full name with specific signature. It may be declared in any assembly and have any accessibility. Design with BCL.

[lambdageek]

Not sure about "all attributes on .NET Core". Some attributes are meaningful to the runtime in a way that editing them may not have the expected effect. (For example FieldOffsetAttribute on a struct with explicit layout).

[tmat]

@lambdageek Yes, we would need to exclude those. Note that these attributes are not real custom attributes. They are emitted as metadata flags. Updated the description to specifically say "custom" attributes.

[lambdageek]

@tmat it's not just pseudo custom attributes. It's also things like UnmanagedCallersOnlyAttribute and SuppressGCTransitionAttribute.

There should be a rude attributes list.

[tmat]

I see. We'll probably need to have a list of special ones.