[CIR] Fix missing return value warning in maybePromoteBoolResult (llvm#142673)

This is NFC and simply adds an llvm_unreachable

Author: mmha

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -104,6 +104,7 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
       return builder.createBoolToInt(value, dstTy);
     if (mlir::isa<cir::BoolType>(dstTy))
       return value;
+    llvm_unreachable("Can only promote integer or boolean types");
   }
 
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