[ORC] Flesh out ExecutorAddress, rename CommonOrcRuntimeTypes header.

Renames CommonOrcRuntimeTypes.h to ExecutorAddress.h and moves ExecutorAddress
into the 'orc' namespace (rather than orc::shared).

Also makes ExecutorAddress a class, adds an ExecutorAddrDiff type and some
arithmetic operations on the pair (subtracting two addresses yields an addrdiff,
adding an addrdiff and an address yields an address).