[clang] [flang] [flang][Driver] Add support for -mllvm -print-pipeline-passes (PR #106141)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> 
> To unblock you, I propose the following:
> 
>     1. Keep this as is, but add a comment saying that this should be a proper compiler option instead of a secret handshake like this. Similar comment should be added in Clang. I will happily approve that.
> 

I have added a comment to both `clang` and `flang` indicating that we should eventually replace `mllvm -print-pipeline-passes` with a proper compiler driver option. I am happy to implement it, but it might have to wait for a bit, and I'd like to get the functionality in for now.

Thanks for the review and suggestions, Andrzej.

https://github.com/llvm/llvm-project/pull/106141