[ORC] Use raw OS handle values, ExecutorAddr for EPC dylib handles.

Updates tpctypes::DylibHandle to be an ExecutorAddr (rather than a uint64_t),
and SimpleExecutorDylibManager to hold and return raw OS handle values (as
ExecutorAddrs) rather than index values into a map of DynamicLibrary instances.

This will allow clients to use EPCGenericDylibManager in contexts where the
existing DynamicLibrary interface is too limited to be used. (e.g. to look up
JIT symbols in a dylib that was loaded with RTLD_LOCAL).