[clang] [clang][CFG] Change child order in Reverse Post Order (RPO) iteration. (PR #80030)

[Yitzhak Mandelbaum via cfe-commits]

ymand wrote:

Gabor, please let me know if you think anyone else should review this change, given that there are assorted clients of the PostCFG view. We think this is a benefit for anyone using a post-order view in a worklist algorithm, but have a hard time anticipating all the potential uses.

https://github.com/llvm/llvm-project/pull/80030