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

[weiwei chen via llvm-commits]

weiweichen wrote:

> 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

I don't want to tear down the whole, I just want to release the memory in this map specifically and dedup them into a centralized place to reduce memory footprint when I have multiple TargetMachines running in parallel in the system. 

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