Fix gcc-8 compilation warning

tests/constructor_stats.h

@@ -180,7 +180,7 @@ public:
                 }
                 }
             }
             }
         }
         }
-        catch (std::out_of_range) {}
+        catch (const std::out_of_range &) {}
         if (!t1) throw std::runtime_error("Unknown class passed to ConstructorStats::get()");
         if (!t1) throw std::runtime_error("Unknown class passed to ConstructorStats::get()");
         auto &cs1 = get(*t1);
         auto &cs1 = get(*t1);
         // If we have both a t1 and t2 match, one is probably the trampoline class; return whichever
         // If we have both a t1 and t2 match, one is probably the trampoline class; return whichever