Fresh constants for configuration initializers

[gtrepta]

fixes #3520

[radumereuta]

I like the solution a lot.
Added some minor comments/questions.

[radumereuta]

I modified ResolveAnonVar to carry over the location information from variables

@@ -104,16 +106,16 @@ public class ResolveAnonVar {
             @Override
             public K apply(KVariable k) {
                 if (ANON_VAR.equals(k)) {
-                    return newDotVariable("");
+                    return newDotVariable("", k);
                 }
                 if (FRESH_ANON_VAR.equals(k)) {
-                    return newDotVariable("?");
+                    return newDotVariable("?", k);
                 }
                 if (FRESH_ANON_CONSTANT.equals(k)) {
-                    return newDotVariable("!");
+                    return newDotVariable("!", k);
                 }
                 if (FRESH_LIST_VAR.equals(k)) {
-                    return newDotVariable("@");
+                    return newDotVariable("@", k);
                 }
                 return super.apply(k);
             }
@@ -121,9 +123,10 @@ public class ResolveAnonVar {
     }
 
     private int counter = 0;
-    KVariable newDotVariable(String prefix) {
+    KVariable newDotVariable(String prefix, K k) {
         KVariable newLabel;
-        Att att = Att().add(Att.ANONYMOUS());
+        Att att = Att().add(Att.ANONYMOUS()).add(Source.class, k.att().get(Source.class))
+                .add(Location.class, k.att().get(Location.class));
         if (prefix.equals("?")) {
             att = att.add(Att.FRESH());
         }

And got the error message to show like this:

[Error] Compiler: Can't resolve fresh configuration variable not of sort Int
  while executing phase "Resolve fresh variables in cell initializers" on sentence at
	<none>
	<none>
	Source(/home/radu/work/test/test.k)
	Location(16,14,16,16)
	16 |	  <freshFoo> !_:Foo </freshFoo>
	   .	             ^~

[radumereuta]

looking great