[ORC] Use a Setup object for SimpleRemoteEPC construction.

SimpleRemoteEPC notionally allowed subclasses to override the
createMemoryManager and createMemoryAccess methods to use custom objects, but
could not actually be subclassed in practice (The construction process in
SimpleRemoteEPC::Create could not be re-used).

Instead of subclassing, this commit adds a SimpleRemoteEPC::Setup class that
can be used by clients to set up the memory manager and memory access members.
A default-constructed Setup object results in no change from previous behavior
(EPCGeneric* memory manager and memory access objects used by default).