[llvm] [Support] Join threads when stopping the ThreadPoolExecutor (PR #166054)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

I'm not sure I follow how this fixes anything. If the `stop` method is not invoked, would the behaviour not be identical to the previous state? If so, how does this fix that?

https://github.com/llvm/llvm-project/pull/166054