function clearCache(type) {
    type._fieldsById = type._fieldsArray = type._oneofsArray = null;
    delete type.encode;
    delete type.decode;
    delete type.verify;
    return type;
}
The registered constructor, if any registered, otherwise a generic constructor.
Assigning a function replaces the internal constructor. If the function does not extend {@link Message} yet, its prototype will be setup accordingly and static methods will be populated. If it already extends {@link Message}, it will just replace the internal constructor.
@name Type#ctor
@type {Constructor<{}>}