value_sett cannot have default copy operator

It uses classes that don't have a default copy operator.  This change is
required for compilation with clang 8.

Author: Daniel Kroening

src/pointer-analysis/value_set.h

@@ -57,6 +57,8 @@ class value_sett
 
   value_sett(const value_sett &other) = default;
 
+  value_sett &operator=(const value_sett &other) = delete;
+
   value_sett &operator=(value_sett &&other)
   {
     values = std::move(other.values);