[clang] [CIR] Upstream support for FlattenCFG switch and SwitchFlatOp (PR #139154)

[via cfe-commits]

Andres-Salamanca wrote:

> This is also missing a end-to-end test that proves we generate switchflatop if we are coming all the way from source code. There are examples in the incubator on how to check if a pass runs (you can use print-before/after) and the IR for it.

In the incubator, the examples use ` %clang_cc1  -mmlir --mlir-print-ir-before=cir-lowering-prepare`, but since that flag isn't available in our current setup, I followed this pipeline instead:

```
clang_cc1 (emit-cir) → cir-opt (cir-flatten-cfg with --mlir-print-ir-before/after)
```


https://github.com/llvm/llvm-project/pull/139154