[lli] Add JITLink in-process debug support

lli aims to provide both, RuntimeDyld and JITLink, as the dynamic linkers/loaders for it's JIT implementations. And they both offer debugging via the GDB JIT interface, which builds on the two well-known symbol names `__jit_debug_descriptor` and `__jit_debug_register_code`. As these symbols must be unique accross the linked executable, we can only define them in one of the libraries and make the other depend on it. OrcTargetProcess is a minimal stub for embedding a JIT client in remote executors. For the moment it seems reasonable to have the definition there and let ExecutionEngine depend on it, until we find a better solution.

This is the second commit for the reviewed patch.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D97339