With the upgrade, BackendPasses was modifying a TargetMachine that was
not used by SimpleCompiler.

Change that by
- using a SimpleCompiler that uses IncrementalJIT::TM;
- moving the TM creation to IncrementalJIT, and giving access to it

This reduces the runtime of https://github.com/root-project/root/issues/11927
to
- before llvm upgrade: 2.69s
- llvm13, without this commit: ???
- llvm13, with this commit: 2.89s
i.e, a slow-down of 7% (that is likely caused by the different emission
mechanism of Orc-v2; to be confirmed...)