[sre] register a canonical MonoReflectionMethod for a methodref token

[sre] register a canonical MonoReflectionMethod for a methodref token

In mono_image_create_token we are passed in a managed reflection object and we
need to create a token for it and optionally register it so that we can resolve
that token back to a runtime structure and managed object later.

For a MonoReflectionMethod object that refers to a method from outside the
current dynamic image, we need to generate a memberref token.  It will be the
same token for the same underlying MonoMethod*.  When we go to register the
token with mono_dynamic_image_register_token we need to pass a managed object.
Unfortunately two MonoReflectionMethod objects could refer to the same
MonoMethod* but differ in their ReflectedType, so just registering the
passed-in MonoReflectionMethod means we could assert in
mono_dynamic_image_register_token because previously we may have registered the
same method via a different ReflectedType.

So what we do is we cal mono_method_get_object_handle with NULL for the
reflected type - which corresponds to just using the DeclaringType of the given
MonoMethod*.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=59364