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

[Matthias Springer via cfe-commits]

matthias-springer wrote:

There have been a series of improvements to the `RegionBranchOpInterface` over the last months. This PR can probably not be cherry-picked in isolation. I suspect that we would also need these:
- https://github.com/llvm/llvm-project/pull/173971
- https://github.com/llvm/llvm-project/pull/173981
- https://github.com/llvm/llvm-project/pull/173983
- https://github.com/llvm/llvm-project/pull/174805

I think it's possible to back-port this, but not sure if it's worth the effort. This PR is not a complete rollback of #161575 and the integration fixes should be quite mechanical.

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