[ORC] Introduce EPCGenericDylibManager / SimpleExecutorDylibManager.

EPCGenericDylibManager provides an interface for loading dylibs and looking up
symbols in the executor, implemented using EPC-calls to functions in the
executor.

SimpleExecutorDylibManager is an executor-side service that provides the
functions used by EPCGenericDylibManager.

SimpleRemoteEPC is updated to use an EPCGenericDylibManager instance to
implement the ExecutorProcessControl loadDylib and lookup methods. In a future
commit these methods will be removed, and clients updated to use
EPCGenericDylibManagers directly.