[clang] [CIR] Upstream initial support for CIR flattening (PR #130648)

[Erich Keane via cfe-commits]

================
@@ -14,9 +14,9 @@ include "mlir/Pass/PassBase.td"
 def CIRFlattenCFG : Pass<"cir-flatten-cfg"> {
   let summary = "Produces flatten CFG";
   let description = [{
-    This pass transforms CIR and inline all the nested regions. Thus,
-    the next post condtions are met after the pass applied:
-    - there is not any nested region in a function body
+    This pass transforms CIR by inlining all the nested regions. Thus,
+    the following condtions are true after the pass applied:
----------------
erichkeane wrote:

```suggestion
    the following conditions are true after the pass applied:
```

https://github.com/llvm/llvm-project/pull/130648