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

[Andy Kaylor 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");
----------------
andykaylor wrote:

There will eventually be three other output types that are handled here and two more that are not. This function is called from within a switch statement that will ensure that the default case here isn't hit. What would be the advantage of an assert over the unreachable call?

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