[marshal-methods] Add marshal methods classification code

[grendello]

Add code which classifies methods selected by JavaCallableWrapperGenerator
and decides whether they are to be registered dynamically or using the
future native code marshal methods.

If a type JavaCallableWrapperGenerator is processing is found to not
have any dynamically registered methods, then the code to register them
is no longer generated.

[jonpryor]

In-progress commit message:

Context: https://github.com/xamarin/xamarin-android/pull/7123
Context: fb94d598bddf7818d841914dd4f565cf5cd0bbac

Instead of having `JavaCallableWrapperGenerator` care about and
collect marshal methods override information, introduce a new
`JavaCallableMethodClassifier` type, and use that as the integration
point:

	public abstract partial class JavaCallableMethodClassifier {
	    public abstract bool ShouldBeDynamicallyRegistered 
	            TypeDefinition topType,
	            MethodDefinition registeredMethod,
	            MethodDefinition implementedMethod,
	            CustomAttribute registerAttribute
	    );
	}

An instance of `JavaCallableMethodClassifier` can now be provided to
the `JavaCallableWrapperGenerator` constructor.  If provided, then
`ShouldBeDynamicallyRegistered()` will be used to control which
methods are added to `__md_methods` and `Runtime.register()`.

If a type `JavaCallableWrapperGenerator` is processing doesn't have
any dynamically registered methods, then the code to register them
is no longer generated.

[jonpryor]

@grendello wrote in the PR description:

If a type JavaCallableWrapperGenerator is processing doesn't have
any dynamically registered methods, then the code to register them
is no longer generated.

What should happen in Debug builds? (Not necessarily relevant here, but it came to mind.) I don't think we want to require LLVM Marshal Methods for Debug builds, so is the plan to provide a JavaCallableMethodClassifier in Debug builds which flags all methods as dynamically registered?

[grendello]

@grendello wrote in the PR description:

If a type JavaCallableWrapperGenerator is processing doesn't have
any dynamically registered methods, then the code to register them
is no longer generated.

What should happen in Debug builds? (Not necessarily relevant here, but it came to mind.) I don't think we want to require LLVM Marshal Methods for Debug builds, so is the plan to provide a JavaCallableMethodClassifier in Debug builds which flags all methods as dynamically registered?

We just don't pass any classifier in this case