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/ast/datatype_decl_plugin.cpp

@@ -231,7 +231,7 @@ namespace datatype {
                 }
                 return s;
             }
-            catch (invalid_datatype) {
+            catch (const invalid_datatype &) {
                 m_manager->raise_exception("invalid datatype");
                 return nullptr;
             }

src/ast/for_each_expr.cpp

@@ -58,7 +58,7 @@ bool has_skolem_functions(expr * n) {
     try {
         for_each_expr(p, n);
     }
-    catch (has_skolem_functions_ns::found) {
+    catch (const has_skolem_functions_ns::found &) {
         return true;
     }
     return false;

src/ast/occurs.cpp

@@ -58,7 +58,7 @@ bool occurs(expr * n1, expr * n2) {
     try {
         quick_for_each_expr(p, n2);
     }
-    catch (occurs_namespace::found) {
+    catch (const occurs_namespace::found &) {
         return true;
     }
     return false;
@@ -69,7 +69,7 @@ bool occurs(func_decl * d, expr * n) {
     try {
         quick_for_each_expr(p, n);
     }
-    catch (occurs_namespace::found) {
+    catch (const occurs_namespace::found &) {
         return true;
     }
     return false;