[ods] Enable getting forward decls allow

[jpienaar]

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).

[jpienaar]

Skipped clang-format check accidentally

[mehdi_amini]

Something that came up internally was to try to move away from the pattern where we define a macro and includes the same file and instead emit multiple files in the first place, would we have one for fwd decl and one for the class decl?

[jpienaar]

In D83727#2149155, @mehdi_amini wrote:

Something that came up internally was to try to move away from the pattern where we define a macro and includes the same file and instead emit multiple files in the first place, would we have one for fwd decl and one for the class decl?

Yes. I mean we'd then effectively have 4 here then 1) fwd decl, 2) class decl, 3) class defn, 4) op list.

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

 
 void OpOperandAdaptorEmitter::emitDef(const Operator &op, raw_ostream &os) {
   OpOperandAdaptorEmitter(op).adaptor.writeDefTo(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) {
       os << formatv(opCommentHeader, op.getQualCppClassName(), "declarations");
       OpOperandAdaptorEmitter::emitDecl(op, os);
       OpEmitter::emitDecl(op, os);
     } else {
       os << formatv(opCommentHeader, op.getQualCppClassName(), "definitions");