fuir: Use sourceRange(), not pos() when missing abstract implemen…

…tation

This is a preparatory patch for PR#3775. Since after #3775, only data form
modules is used and the position of an Expr is lost, only the source range is
left, the test output would change.

src/dev/flang/fuir/FUIR.java

@@ -2085,7 +2085,7 @@ public SourcePosition sitePos(int s)
     if (s != NO_SITE)
       {
         var e = getExpr(s);
-        result = (e instanceof Expr expr) ? expr.pos() :
+        result = (e instanceof Expr expr) ? expr.sourceRange() :
                  (e instanceof Clazz z)   ? z._type.declarationPos()  /* implicit assignment to argument field */
                                           : null;
       }