Add missing root for method instances used in opaque closure
Usually the rooting path for method instances is something like this:
Module -> DataType -> TypeName -> MethodTable -> Method -> MethodInstance
so these MethodInstances are effectively globally rooted. However, the
Method(instances) inside opaque closures do not have this rooting path
and can be deleted. Unfortunately, if we codegen'ed these opaque closures,
we do have a reference to the method instance in our global debuginfo
tables. This was causing crashes in cases where the oc was gc'ed
before a stack trace could be printed. Fix that by adding any method
instance that needs to be rooted to the global roots table.
