[llvm] Overhaul the TargetMachine and LLVMTargetMachine Classes (PR #111234)

[Matin Raayai via llvm-commits]

matinraayai wrote:

@arsenm @paperchalice any suggestions on what I should change here? I can either:
1. Make `CodeGenTargetMachine` behave the same way it did before, and add a `dyn_cast` option to it to confirm whether a `TargetMachine` was implemented with code gen support; or
2. Keep the PR the way it is; i.e. `CodeGenTargetMachine` is just a set of function implementations a `TargetMachine` can inherit. 

https://github.com/llvm/llvm-project/pull/111234