[mlir] Add a string type

PiperOrigin-RevId: 206977161
diff --git a/lib/IR/Builders.cpp b/lib/IR/Builders.cpp
index 9ba75e4..199f35d 100644
--- a/lib/IR/Builders.cpp
+++ b/lib/IR/Builders.cpp
@@ -47,6 +47,8 @@
 
 OtherType *Builder::getTFControlType() { return Type::getTFControl(context); }
 
+OtherType *Builder::getTFStringType() { return Type::getTFString(context); }
+
 IntegerType *Builder::getIntegerType(unsigned width) {
   return Type::getInteger(width, context);
 }