Fix symbol clean-up

Replace old values (when leaving a block) by default (constructor)
values. This was buggy since old values were replaced by a
`std::monostate` which led to inconsistent state of local variables.

src/language/utils/SymbolTable.hpp

@@ -267,7 +267,12 @@ class SymbolTable
   clearValues()
   {
     for (auto& symbol : m_symbol_list) {
-      symbol.attributes().value() = DataVariant{};
+      std::visit(
+        [](auto&& value) {
+          using T = std::decay_t<decltype(value)>;
+          value   = T{};
+        },
+        symbol.attributes().value());
     }
   }