[ORC] Add a shared-memory based orc::MemoryMapper.

This is an implementation of orc::MemoryMapper that maps shared memory
pages in both executor and controller process and writes directly to
them avoiding transferring content over EPC. All allocations are properly
deinitialized automatically on the executor side at shutdown by the
ExecutorSharedMemoryMapperService.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D128544