[ods] Enable getting forward decls allow

Summary: Currently forward decls are included with all the op classes. But there are cases (say when splitting up headers) where one wants the forward decls but not all the classes. Add an option to enable this. This does not change any current behavior (some further refactoring is probably due here).

Differential Revision: https://reviews.llvm.org/D83727

Author: jpienaar

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

@@ -2129,15 +2129,19 @@ void OpOperandAdaptorEmitter::emitDef(const Operator &op, raw_ostream &os) {
 // Emits the opcode enum and op classes.
 static void emitOpClasses(const std::vector<Record *> &defs, raw_ostream &os,
                           bool emitDecl) {
-  IfDefScope scope("GET_OP_CLASSES", os);
   // First emit forward declaration for each class, this allows them to refer
   // to each others in traits for example.
   if (emitDecl) {
+    os << "#if defined(GET_OP_CLASSES) || defined(GET_OP_FWD_DEFINES)\n";
+    os << "#undef GET_OP_FWD_DEFINES\n";
     for (auto *def : defs) {
       Operator op(*def);
       os << "class " << op.getCppClassName() << ";\n";
     }
+    os << "#endif\n\n";
   }
+
+  IfDefScope scope("GET_OP_CLASSES", os);
   for (auto *def : defs) {
     Operator op(*def);
     if (emitDecl) {