[llvm] [Backend] Add clearSubtargetMap API for TargetMachine. (PR #112383)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> They are mutable and being modified mostly in the same pattern [here](https://github.com/llvm/llvm-project/blob/b0a25468faca32d9db4d35e56fb120ed4eaeec09/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp#L467). Is there a plan to change that to make TargetMachine fully stateless as well?

No, I'm not aware of one. The other problematic piece is TargetOptions

https://github.com/llvm/llvm-project/pull/112383