Catch exceptions by const-reference

Exceptions caught by value incur needless cost in C++, most of them can
be caught by const-reference, especially as nearly none are actually
used. This could allow compiler generate a slightly more efficient code.

src/tactic/arith/bv2real_rewriter.cpp

@@ -89,7 +89,7 @@ bool bv2real_util::contains_bv2real(expr* e) const {
     try {
         for_each_expr(p, e);
     }
-    catch (contains_bv2real_proc::found) {
+    catch (const contains_bv2real_proc::found &) {
         return true;
     }
     return false;

src/tactic/arith/fix_dl_var_tactic.cpp

@@ -226,7 +226,7 @@ class fix_dl_var_tactic : public tactic {
                 }
                 return most_occs();
             }
-            catch (failed) {
+            catch (const failed &) {
                 return nullptr;
             }
         }

src/tactic/arith/lia2pb_tactic.cpp

@@ -156,7 +156,7 @@ class lia2pb_tactic : public tactic {
                 }
                 return true;
             }
-            catch (failed) {
+            catch (const failed &) {
                 return false;
             }
         }

src/tactic/arith/pb2bv_tactic.cpp

@@ -1034,7 +1034,7 @@ struct is_pb_probe : public probe {
             
             return true;
         }
-        catch (pb2bv_tactic::non_pb) {
+        catch (const pb2bv_tactic::non_pb &) {
             return false;
         }
     }