[MLIR] Add ODS support for generating helpers for dialect (discardable) attributes (#77024)

This is a new ODS feature that allows dialects to define a list of
key/value pair representing an attribute type and a name.
This will generate helper classes on the dialect to be able to
manage discardable attributes on operations in a type safe way.

For example the `test` dialect can define:

```
  let discardableAttrs = (ins
     "mlir::IntegerAttr":$discardable_attr_key,
  );
```

And the following will be generated in the TestDialect class:

```
   /// Helper to manage the discardable attribute `discardable_attr_key`.
    class DiscardableAttrKeyAttrHelper {
      ::mlir::StringAttr name;
    public:
      static constexpr ::llvm::StringLiteral getNameStr() {
        return "test.discardable_attr_key";
      }
      constexpr ::mlir::StringAttr getName() {
        return name;
      }

      DiscardableAttrKeyAttrHelper(::mlir::MLIRContext *ctx)
        : name(::mlir::StringAttr::get(ctx, getNameStr())) {}

     mlir::IntegerAttr getAttr(::mlir::Operation *op) {
       return op->getAttrOfType<mlir::IntegerAttr>(name);
     }
     void setAttr(::mlir::Operation *op, mlir::IntegerAttr val) {
       op->setAttr(name, val);
     }
     bool isAttrPresent(::mlir::Operation *op) {
       return op->hasAttrOfType<mlir::IntegerAttr>(name);
     }
     void removeAttr(::mlir::Operation *op) {
       assert(op->hasAttrOfType<mlir::IntegerAttr>(name));
       op->removeAttr(name);
     }
   };
   DiscardableAttrKeyAttrHelper getDiscardableAttrKeyAttrHelper() {
     return discardableAttrKeyAttrName;
   }
```

User code having an instance of the TestDialect can then manipulate this
attribute on operation using:

```
  auto helper = testDialect.getDiscardableAttrKeyAttrHelper();

  helper.setAttr(op, value);
  helper.isAttrPresent(op);
  ...
```