[ORC] Replace MachOJITDylibInitializers::SectionExtent with ExecutorAddressRange

MachOJITDylibInitializers::SectionExtent represented the address range of a
section as an (address, size) pair. The new ExecutorAddressRange type
generalizes this to an address range (for any object, not necessarily a section)
represented as a (start-address, end-address) pair.

The aim is to express more of ORC (and the ORC runtime) in terms of simple types
that can be serialized/deserialized via SPS. This will simplify SPS-based RPC
involving arguments/return-values of these types.