[mlir] Add UnitParameter to indicate the presence of a flag

[jdd]

Summary

A type or attribute parameter which wraps UnitAttr with no printing. Helpful in
conjunction with the declarative assembly format's optional parens.

[jdd]

Will create a PR.

mlir/include/mlir/IR/AttrTypeBase.td

class AttributeSelfTypeParameter<string desc,
                                  string derivedType = "::mlir::Type",
                                  string typeBuilder = ""> :
     AttrOrTypeParameter<derivedType, desc> {
   let defaultValue = !if(!and(!empty(typeBuilder),
                               !eq(derivedType, "::mlir::Type")),
                          "::mlir::NoneType::get($_ctxt)", typeBuilder);
 }
 
+// Used to indicate the presence of a keyword in asm. Should be used within
+// optional parens in the declarative assembly format.
+def UnitParameter : AttrOrTypeParameter<"::mlir::UnitAttr", "boolean flag"> {
+  let printer = "";
+  let parser = "::mlir::UnitAttr::get($_parser.getContext())";
+  let defaultValue = "::mlir::UnitAttr()";
+}
+
 //===----------------------------------------------------------------------===//
 // ArrayOfAttr
 //===----------------------------------------------------------------------===//
 
 /// This class defines an attribute that contains an array of elements. The
 /// elements can be any type, but if they are attributes, the nested elements
 /// are parsed and printed using the custom attribute syntax.
 class ArrayOfAttr<Dialect dialect, string attrName, string attrMnemonic,