Revert "[SCCP] Remove forcedconstant, go to overdefined instead"

This causes a crash for the reproducer below

 enum { a };
 enum b { c, d };
 e;
 static _Bool g(struct f *h, enum b i) {
   i &&j();
   return a;
 }
 static k(char h, enum b i) {
   _Bool l = g(e, i);
   l;
 }
 m(h) {
   k(h, c);
   g(h, d);
 }

This reverts commit aadb635e.