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

[weiwei chen via llvm-commits]

weiweichen wrote:

> > I don't want to tear down the whole TargetMachine, 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.
> 
> Why do you need multiple TargetMachines from the same target to compile in parallel?

To parallelize llvm compilation for more efficient compilation time. 

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