Internals: Use C++11 = default where obvious. No functional change intended.

src/V3Clock.cpp

@@ -454,7 +454,7 @@ public:
         // easily without iterating through the tree.
         nodep->evalp(m_evalFuncp);
     }
-    virtual ~ClockVisitor() override {}
+    virtual ~ClockVisitor() override = default;
 };
 
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