[clang] [CIR] Add framework for CIR to LLVM IR lowering (PR #124650)

[Aaron Ballman via cfe-commits]

================
@@ -7,42 +7,56 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/CIR/FrontendAction/CIRGenAction.h"
-#include "clang/CIR/CIRGenerator.h"
-#include "clang/Frontend/CompilerInstance.h"
-
 #include "mlir/IR/MLIRContext.h"
 #include "mlir/IR/OwningOpRef.h"
+#include "clang/CIR/CIRGenerator.h"
+#include "clang/CIR/LowerToLLVM.h"
+#include "clang/CodeGen/BackendUtil.h"
+#include "clang/Frontend/CompilerInstance.h"
+#include "llvm/IR/Module.h"
 
 using namespace cir;
 using namespace clang;
 
 namespace cir {
 
+static BackendAction
+getBackendActionFromOutputType(CIRGenAction::OutputType Action) {
+  switch (Action) {
+  case CIRGenAction::OutputType::EmitLLVM:
+    return BackendAction::Backend_EmitLL;
+  default:
+    llvm_unreachable("Unsupported action");
----------------
AaronBallman wrote:

It boils down to invariants. There is no `BackendAction::Backend_EmitCIR`, so passing in `EmitCIR` as an output type seems like it would always be a mistaken use of the API, so an assert would make more sense, leaving the unreachable for nonsense cases like passing `(CIRGenAction::OutputType)12`.

https://github.com/llvm/llvm-project/pull/124650