PackageCMO: fix serializability check for keypath
When a keypath instruction was checked for serializability, its referenced function was
sometimes incorrectly deemed serializable when its referenced method had package or
public access level. This resulted in incorrectly serializing a function that dynamically
accesses a property on a generic type using key path. This PR fixes the issue by skipping
the access level check if the referenced function is determined to be un-serializable.
Resolves rdar://142950306
