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

[Matt Arsenault via llvm-commits]

arsenm wrote:

What's broken is the experimental targets, which in theory you shouldn't have to worry about. You still get the build failure notifications, so this is a dumb system. We should either not notify on experimental target builds, or get rid of the concept entirely 

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