Add is*() functions for more types.

Test: compiles
Change-Id: Ibeba6c814ac17192481935a067b7c835c4008dc7
diff --git a/StringType.h b/StringType.h
index d161fdb..085ec11 100644
--- a/StringType.h
+++ b/StringType.h
@@ -27,6 +27,8 @@
 
     void addNamedTypesToSet(std::set<const FQName> &set) const override;
 
+    bool isString() const override;
+
     std::string getCppType(
             StorageMode mode,
             bool specifyNamespaces) const override;