Add ::mlir:: prefix to Attribute type in the generated code.

mlir/tools/mlir-tblgen/OpFormatGen.cpp

for (const NamedAttribute &namedAttr : op.getAttributes()) {
     if (!attr.isDerivedAttr() && attr.hasDefaultValue()) {
       const StringRef &name = namedAttr.name;
       FmtContext fctx;
       fctx.withBuilder("odsBuilder");
       std::string defaultValue = std::string(
           tgfmt(attr.getConstBuilderTemplate(), &fctx, attr.getDefaultValue()));
       body << "  {\n";
       body << "     ::mlir::Builder odsBuilder(getContext());\n";
-      body << "     Attribute attr = " << op.getGetterName(name) << "Attr();\n";
+      body << "     ::mlir::Attribute attr = " << op.getGetterName(name)
+           << "Attr();\n";
       body << "     if(attr && (attr == " << defaultValue << "))\n";
       body << "       elidedAttrs.push_back(\"" << name << "\");\n";
       body << "  }\n";
     }
   }
   body << "  _odsPrinter.printOptionalAttrDict"
        << (withKeyword ? "WithKeyword" : "")
        << "((*this)->getAttrs(), elidedAttrs);\n";