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

[Bruno Cardoso Lopes via cfe-commits]

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@@ -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";
+  let description = [{
+    This pass does preparation work for LLVM lowering. For example, it may
+    expand the global variable initialziation in a more ABI-friendly form.
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bcardosolopes wrote:

Please make the description a bit more generic (not LLVM specific) since it also covers the core dialect path in the incubator. 

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