[Orc] Only unmap shared memory in controller process destructor

By the time SharedMemoryMapper destructor is called, the RPC
connection is no longer available causing the release() call to
always fail. Instead at this point only shared memory regions
can be unmapped safely.

Deinitializers are called and mapped memory is released at the
executor side by ExecutorSharedMemoryMapperService::shutdown()
instead. Memory can also be released earlier by calling release()
earlier before RPC connection is closed.

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