[clang] [CIR] Add transform test for cir-flatten-cfg (PR #130861)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I don't have concerns, but am not the best person whose approval should count here.  I have questions/comments on the scope of the flatten pass (that is, what 'while we are here' type stuff it should do), but I'm again not really an authority on these.

https://github.com/llvm/llvm-project/pull/130861