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

[Matin Raayai via llvm-commits]

matinraayai wrote:

@aeubanks I put the RFC in my commit message. I'm also putting it here for reference: https://discourse.llvm.org/t/rfc-reworking-the-targetmachine-interface/82478
I changed the name to `CodeGenTargetMachineImpl` as I wanted to emphasize that it should not be used directly in code unless to inherit its methods.

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