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/sat/sat_solver.cpp

@@ -1147,7 +1147,7 @@ namespace sat {
 
             }
         }
-        catch (abort_solver) {
+        catch (const abort_solver &) {
             m_reason_unknown = "sat.giveup";
             return l_undef;
         }