[Orc] Provide correct Reservation address for slab allocations

When slab allocator is used, the MappingBase is not necessarily
the same as the original reservation base as the allocation could
be a part of the whole reservation.

In this case the original reservation address needs to be passed to
ExecutorSharedMemoryMapperService to associate the new allocation
with the original reservation.

Differential Revision: https://reviews.llvm.org/D132313