[mlir] [polly] [mlir][polly][llvm-lit] Fixed logic for turning on external shell in lit (PR #106458)

[Connie Zhu via llvm-commits]

connieyzhu wrote:

> Yeah that’s likely the best idea. Or at least a tracking issue.

@boomanaiden154 @ilovepi I added TODOs in my most recent commit and created an [issue](https://github.com/llvm/llvm-project/issues/106636) for this.

https://github.com/llvm/llvm-project/pull/106458