Explorer: fix variables not cleaned up if declared before unformed (#2544)

Fix variables not being properly cleaned up if declared before an unformed variable.

### Details

Currently the following example / test
```
package ExplorerTest api;

class A {
  var i: i32;
  destructor[self: Self] {
    Print("Destructor A {0}", self.i);
  }
}

fn Main() -> i32 {
  var a0: A;
  var a1: A = {.i = 1};
  var a2: A;
  var a3: A = {.i = 3};
  return 0;
}
```

prints
```
Destructor A 3
```

instead of 

```
Destructor A 3
Destructor A 1
```

This PR fixes the issue in the `CleanUp` logic.

explorer/testdata/unformed/destroy_formed_only.carbon

@@ -0,0 +1,27 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{explorer-run}
+// RUN: %{explorer-run-trace}
+// CHECK:STDOUT: Destructor A 3
+// CHECK:STDOUT: Destructor A 1
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+class A {
+  var i: i32;
+  destructor[self: Self] {
+    Print("Destructor A {0}", self.i);
+  }
+}
+
+fn Main() -> i32 {
+  var a0: A;
+  var a1: A = {.i = 1};
+  var a2: A;
+  var a3: A = {.i = 3};
+  return 0;
+}