[llvm] [NewPM][CodeGen] add TargetPassConfig like API (PR #70906)

[Nishant Mohan via llvm-commits]

lelouch-17 wrote:

> Hi, thanks for your interest in this! Currently the migration work is still in early stage, building pipeline for a specific backend might too early.

We are creating a dummy pipeline of sort for AMDGPU right now. 

>  If you really need it, I will open another pull request to support it.

It would be very helpful if you can do that. 

https://github.com/llvm/llvm-project/pull/70906