Fix some (clang) compiler warnings

lib/checkunusedvar.cpp

@@ -93,6 +93,9 @@ public:
         }
 
     private:
+        /** No implementation */
+        ScopeGuard& operator=(const ScopeGuard &);
+
         Variables & _guarded;
         bool _insideLoop;
     };

lib/executionpath.h

@@ -35,7 +35,7 @@ class SymbolDatabase;
 class CPPCHECKLIB ExecutionPath {
 private:
     /** No implementation */
-    void operator=(const ExecutionPath &);
+    ExecutionPath& operator=(const ExecutionPath &);
 
 protected:
     Check * const owner;

lib/mathlib.cpp

@@ -423,7 +423,6 @@ bool MathLib::isValidSuffix(std::string::const_iterator it, std::string::const_i
             break;
         default:
             return false;
-            break;
         }
     }
     return ((state == SUFFIX_U)   ||