[flang] Make SetLength TODO explicit in HLFIR

[jeanPerier]

Summary

evaluate::SetLength is the last intrinsic binary operation
that needs to be lowered to HLFIR. It will require an hlfir.set_length
op or hlfir.as_expr to convert the result to an expression. Add a TODO
for now.

Depends on D139180

flang/lib/Lower/ConvertExprToHLFIR.cpp

private:
   mlir::Location loc;
 };
 
 //===--------------------------------------------------------------------===//
 // Binary Operation implementation
 //===--------------------------------------------------------------------===//
 
 template <typename T>
-struct BinaryOp {
-  static hlfir::EntityWithAttributes gen(mlir::Location loc,
-                                         fir::FirOpBuilder &builder, const T &,
-                                         hlfir::Entity lhs, hlfir::Entity rhs) {
-    TODO(loc, "binary op implementation in HLFIR");
-  }
-};
+struct BinaryOp {};
 
 #undef GENBIN
 #define GENBIN(GenBinEvOp, GenBinTyCat, GenBinFirOp)                           \
   template <int KIND>                                                          \
   struct BinaryOp<Fortran::evaluate::GenBinEvOp<Fortran::evaluate::Type<       \
       Fortran::common::TypeCategory::GenBinTyCat, KIND>>> {                    \
     using Op = Fortran::evaluate::GenBinEvOp<Fortran::evaluate::Type<          \
         Fortran::common::TypeCategory::GenBinTyCat, KIND>>;                    \
static hlfir::EntityWithAttributes gen(mlir::Location loc,
                                          const Op &op, hlfir::Entity lhs,
                                          hlfir::Entity rhs) {
     mlir::Value res =
         fir::factory::Complex{builder, loc}.createComplex(KIND, lhs, rhs);
     return hlfir::EntityWithAttributes{res};
   }
 };
 
+template <int KIND>
+struct BinaryOp<Fortran::evaluate::SetLength<KIND>> {
+  using Op = Fortran::evaluate::SetLength<KIND>;
+  static hlfir::EntityWithAttributes gen(mlir::Location loc,
+                                         fir::FirOpBuilder &builder,
+                                         const Op &op, hlfir::Entity lhs,
+                                         hlfir::Entity rhs) {
+    TODO(loc, "SetLength lowering to HLFIR");
+  }
+};
+
 /// Lower Expr to HLFIR.
 class HlfirBuilder {
 public:
   HlfirBuilder(mlir::Location loc, Fortran::lower::AbstractConverter &converter,
                Fortran::lower::SymMap &symMap,
                Fortran::lower::StatementContext &stmtCtx)
       : converter{converter}, symMap{symMap}, stmtCtx{stmtCtx}, loc{loc} {}