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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I think this is too specific. What is the use case for this? If you're tearing down the TargetMachine, I would expect to release everything. If you're reusing the TargetMachine, I would also expect to just leave everything as it is

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