[mlir] Don't emit forward declaration for user defined storage classes

Currently DefGen::emitDecl always emits forward declarations of storage classes even for user define ones, which makes it difficult to use template class directly in ODS. This patch changes `DefGen` not to emit forward decl when `genStorageClass` is false.

Original discussion: https://discourse.llvm.org/t/use-template-classes-as-user-defined-storage-classes/72015

Reviewed By: mehdi_amini, rriddle

Differential Revision: https://reviews.llvm.org/D155225

Author: uenoku

flang/include/flang/Optimizer/Dialect/FIRType.h

@@ -22,6 +22,11 @@ namespace fir {
 class FIROpsDialect;
 class KindMapping;
 using KindTy = unsigned;
+
+namespace detail {
+struct RecordTypeStorage;
+} // namespace detail
+
 } // namespace fir
 
 //===----------------------------------------------------------------------===//

mlir/include/mlir/IR/BuiltinAttributes.h

@@ -31,6 +31,12 @@ class Location;
 class Operation;
 class RankedTensorType;
 
+namespace detail {
+struct DenseIntOrFPElementsAttrStorage;
+struct DenseStringElementsAttrStorage;
+struct StringAttrStorage;
+} // namespace detail
+
 //===----------------------------------------------------------------------===//
 // Elements Attributes
 //===----------------------------------------------------------------------===//

mlir/include/mlir/IR/BuiltinTypes.h

@@ -32,6 +32,12 @@ class RankedTensorType;
 class StringAttr;
 class TypeRange;
 
+namespace detail {
+struct FunctionTypeStorage;
+struct IntegerTypeStorage;
+struct TupleTypeStorage;
+} // namespace detail
+
 //===----------------------------------------------------------------------===//
 // FloatType
 //===----------------------------------------------------------------------===//

mlir/tools/mlir-tblgen/AttrOrTypeDefGen.cpp

@@ -67,7 +67,7 @@ class DefGen {
   DefGen(const AttrOrTypeDef &def);
 
   void emitDecl(raw_ostream &os) const {
-    if (storageCls) {
+    if (storageCls && def.genStorageClass()) {
       NamespaceEmitter ns(os, def.getStorageNamespace());
       os << "struct " << def.getStorageClassName() << ";\n";
     }