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/muz/spacer/spacer_context.cpp

@@ -2737,7 +2737,7 @@ lbool context::solve(unsigned from_lvl)
             // }
         }
         VERIFY (validate ());
-    } catch (unknown_exception)
+    } catch (const unknown_exception &)
     {}
 
     if (m_last_result == l_true) {

src/muz/spacer/spacer_farkas_learner.cpp

@@ -95,7 +95,7 @@ bool farkas_learner::is_pure_expr(func_decl_set const& symbs, expr* e, ast_manag
     is_pure_expr_proc proc(symbs, m);
     try {
         for_each_expr(proc, e);
-    } catch (is_pure_expr_proc::non_pure) {
+    } catch (const is_pure_expr_proc::non_pure &) {
         return false;
     }
     return true;

src/muz/spacer/spacer_iuc_proof.cpp

@@ -88,7 +88,7 @@ bool iuc_proof::is_core_pure(expr* e) const
     try {
         for_each_expr(proc, e);
     }
-    catch (is_pure_expr_proc::non_pure)
+    catch (const is_pure_expr_proc::non_pure &)
     {return false;}
 
     return true;

src/muz/spacer/spacer_manager.cpp

@@ -247,7 +247,7 @@ bool has_zk_const(expr *e){
     try {
         for_each_expr(p, e);
     }
-    catch (has_zk_const_ns::found) {
+    catch (const has_zk_const_ns::found &) {
         return true;
     }
     return false;

src/muz/spacer/spacer_quant_generalizer.cpp

@@ -120,7 +120,7 @@ struct index_lt_proc : public std::binary_function<app*, app *, bool> {
             for (expr *e : v)
                 quick_for_each_expr(fn, visited, e);
         }
-        catch (has_nlira_functor::found ) {
+        catch (const has_nlira_functor::found &) {
             return true;
         }
         return false;

src/muz/spacer/spacer_util.cpp

@@ -891,7 +891,7 @@ namespace {
             for_each_expr(cs, fml);
             return false;
         }
-        catch(found) {
+        catch(const found &) {
             return true;
         }
     }