[ORC] Add Shared/OrcRTBridge, and TargetProcess/OrcRTBootstrap.

This is a small first step towards reorganization of the ORC libraries:

Declarations for types and function names (as strings) to be found in the
"ORC runtime bootstrap" set are moved into OrcRTBridge.h / OrcRTBridge.cpp.

The current implementation of the "ORC runtime bootstrap" functions is moved
into OrcRTBootstrap.h and OrcRTBootstrap.cpp. It is likely that this code will
eventually be moved into ORT-RT proper (in compiler RT).

The immediate goal of this change is to make these bootstrap functions usable
for clients other than SimpleRemoteEPC/SimpleRemoteEPCServer. The first planned
client is a new RuntimeDyld::MemoryManager that will run over EPC, which will
allow us to remove the old OrcRemoteTarget code.