[mlir] Add a string type

PiperOrigin-RevId: 206977161
diff --git a/lib/Parser/Parser.cpp b/lib/Parser/Parser.cpp
index a468c9e..24bda89 100644
--- a/lib/Parser/Parser.cpp
+++ b/lib/Parser/Parser.cpp
@@ -309,6 +309,9 @@
   case Token::kw_tf_control:
     consumeToken(Token::kw_tf_control);
     return builder.getTFControlType();
+  case Token::kw_tf_string:
+    consumeToken(Token::kw_tf_string);
+    return builder.getTFStringType();
   }
 }