[clang] [CIR] Upstream the basic structure of LoweringPrepare pass (PR #148545)

[Bruno Cardoso Lopes via cfe-commits]

================
@@ -72,4 +72,14 @@ def CIRFlattenCFG : Pass<"cir-flatten-cfg"> {
   let dependentDialects = ["cir::CIRDialect"];
 }
 
+def LoweringPrepare : Pass<"cir-lowering-prepare"> {
+  let summary = "Preparation work before lowering to LLVM dialect";
----------------
bcardosolopes wrote:

`Preparation work before lowering to LLVM dialect` -> `Lower to more fine-grained CIR operations before lowering to other dialects`

https://github.com/llvm/llvm-project/pull/148545