libexpr: handle debug trace frames as parent pointer list

this also fixes a debugger bug where leaving the debugger does not clean
up old debugger state completely. in such cases the fake frame withFrame
created was left behind after the corresponding caller frame was unwound

Change-Id: I45adcd116276b03b2f87076518c9eae6fe844e06

tests/functional/repl_characterization/data/debug_frames.test

@@ -71,3 +71,43 @@ and resume execution
 the debugger is once again disabled
   nix-repl> :c
   error: unknown command ':c'
+
+leaving the debugger from a toplevel error and entering it again doesn't leave old frames visible
+  nix-repl> with {}; a
+  error: undefined variable 'a'
+         at «string»:1:10:
+              1| with {}; a
+               |          ^
+
+  nix-repl> :s
+  error: undefined variable 'a'
+         at «string»:1:10:
+              1| with {}; a
+               |          ^
+
+  nix-repl> with {}; b
+  error: undefined variable 'b'
+         at «string»:1:10:
+              1| with {}; b
+               |          ^
+
+  nix-repl> :bt
+
+  0: error: undefined variable 'b'
+  «string»:1:10
+
+       1| with {}; b
+        |          ^
+
+  1: error: Fake frame for debugging purposes
+  «string»:1:10
+
+       1| with {}; b
+        |          ^
+
+exiting from here prints the error
+  nix-repl> :q
+  error: undefined variable 'b'
+         at «string»:1:10:
+              1| with {}; b
+               |          ^