[clang] [flang] [mlir] [flang] Add support for -mprefer-vector-width=<value> (PR #141380)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> Seems ok now: "Changes can be cleanly merged."
> 
> If it's still not available, I may have messed something up on my side. I'll review the GitHub docs to see if I missed something...

It's probably fine, but I'd like to give the buildkites a chance to finish before I merge it.

https://github.com/llvm/llvm-project/pull/141380