[Kaleidoscope][BuildingAJIT] Add code for Chapter 5 - remote JITing.

This chapter demonstrates lazily JITing from ASTs with the expressions being
executed on a remote machine via a TCP connection. It needs some polish, but is
substantially complete.

Currently x86-64 SysV ABI (Darwin and Linux) only, but other architectures
can be supported by changing the server code to use alternative ABI support
classes from llvm/include/llvm/ExecutionEngine/Orc/OrcABISupport.h.

llvm-svn: 271193