message: Add bool constant attributes.
diff: @@ -780,6 +780,8 @@ class ConstantAttr<Attr attribute, string val> : AttrConstraint<
  }
  class ConstF32Attr<string val> : ConstantAttr<F32Attr, val>;
  +def ConstBoolAttrFalse : ConstantAttr<BoolAttr, "false">;
  +def ConstBoolAttrTrue : ConstantAttr<BoolAttr, "true">;
  //===----------------------------------------------------------------------===//
  // Common attribute constraints
  