[llvm] [AMDGPU] Move AMDGPUCodeGenPassBuilder into AMDGPUTargetMachine(NFC) (PR #103720)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I can't decide between this and #102865. It was nice to have this in a separate file. But I guess TargetPassConfig was already here (for some reason), so it makes sense to keep both pass manager implementations together 

https://github.com/llvm/llvm-project/pull/103720