Internals: Favor const for map keys. No functional change intended.

src/V3CUse.cpp

@@ -41,7 +41,7 @@ private:
     // MEMBERS
     AstNodeModule* m_modInsertp;  // Current module to insert AstCUse under
     typedef std::pair<VUseType, string> UseString;
-    std::map<UseString, AstCUse*> m_didUse;  // What we already used
+    std::map<const UseString, AstCUse*> m_didUse;  // What we already used
 
     // NODE STATE
     // Entire netlist: