streamline pb solver interface and naming after removal of xor

src/sat/sat_simplifier.h

@@ -31,6 +31,10 @@ Revision History:
 #include "util/statistics.h"
 #include "util/params.h"
 
+namespace pb {
+    class solver;
+}
+
 namespace sat {
     class solver;
 
@@ -50,7 +54,7 @@ namespace sat {
     };
 
     class simplifier {
-        friend class ba_solver;
+        friend class pb::solver;
         friend class elim_vars;
         solver &               s;
         unsigned               m_num_calls;