[clang] [CIR] Implement flattening for cleanup scopes with multiple exits (PR #180627)

[Henrich Lauko via cfe-commits]

https://github.com/xlauko commented:

With multi-exit now working, `enclosedByCleanupScopeWithMultipleExits` can never return `true` when the switch/loop patterns run (cleanup patterns always process first due to PostOrder + the new nested cleanup guard). The dead code should be probably removed?

https://github.com/llvm/llvm-project/pull/180627