[mlir] Add a string type

PiperOrigin-RevId: 206977161
diff --git a/lib/IR/AsmPrinter.cpp b/lib/IR/AsmPrinter.cpp
index 9a94d66..f881872 100644
--- a/lib/IR/AsmPrinter.cpp
+++ b/lib/IR/AsmPrinter.cpp
@@ -303,6 +303,9 @@
   case Type::Kind::TFControl:
     os << "tf_control";
     return;
+  case Type::Kind::TFString:
+    os << "tf_string";
+    return;
 
   case Type::Kind::Integer: {
     auto *integer = cast<IntegerType>(type);