[llvm] [FileCheck] --dump-input: render search ranges with `{` ... `}` brackets (PR #194251)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Thank you for continuing to pursue this. It sounds like we agree on the general direction even if we have not converged on some details.
> 
> However, I feel like you are being forced to reproduce work I have already done. If you would like to continue to get more involved in the FileCheck implementation, that is fine, and I will continue to review your patches. But if your goal is purely to see these issues addressed, and if you are willing to review, it might prove more efficient for me to update my patches and post them. I must warn that they start with some implementation refactoring (I [posted a version previously](https://reviews.llvm.org/D126673) but never got back to it), but I think that is worthwhile to facilitate these and other improvements while keeping the design clean.

The goal is to get the `--dump-input` usability problem fixed, not who lands the patch. Please post your version. I am happy to review and abandon this patch. If any piece of this patch is useful, feel free to take them. 

https://github.com/llvm/llvm-project/pull/194251