[POST MCJIT] Deprecate and remove TargetMachine::addPassesToEmitMC.

From its comment `TargetMachine::addPassesToEmitMC` was intended to be JIT-specific:
```c++
/// Add passes to the specified pass manager to get machine code emitted with
/// the MCJIT. This method returns true if machine code is not supported. It
/// fills the MCContext Ctx pointer which can be used to build custom
/// MCStreamer.
```

Once MCJIT is deprecated and removed we should also deprecate and then remove this method.