[clang] [flang] [mlir] [mlir][Interfaces][NFC] Simplify and align `RegionSuccessor` design / API (PR #174945)

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mtsamis wrote:

> Maybe we can just mention in the release notes how to migrate the code?

That would probably be helpful. As you pointed out there are a lot of commits that change `RegionSuccessor` APIs and it is not always clear what to use.

https://github.com/llvm/llvm-project/pull/174945