AAPT2: Better debugging output

Test: make aapt2_tests
Change-Id: I7778b773201381538dc1f2e376abee4eb33e44c0
diff --git a/tools/aapt2/ConfigDescription.h b/tools/aapt2/ConfigDescription.h
index c1d0e10..f719552 100644
--- a/tools/aapt2/ConfigDescription.h
+++ b/tools/aapt2/ConfigDescription.h
@@ -64,6 +64,8 @@
   // Returns the BCP-47 language tag of this configuration's locale.
   std::string GetBcp47LanguageTag(bool canonicalize = false) const;
 
+  std::string to_string() const;
+
   /**
    * A configuration X dominates another configuration Y, if X has at least the
    * precedence of Y and X is strictly more general than Y: for any type defined