parser-state: fix attribute merging (backport #11294)

[mergify]

Motivation

Fixes #11268.

Context

This should be non-breaking. As a side effect, it makes some duplicate attribute error messages nicer (later occurrences point back to previous occurrences rather than the other way around).

I'm working at the edge of my proficiency with C++ here, so please take extra care to verify that how I'm handling pointers and memory management is sensible, because I'm not confident that I'm doing it right.

Priorities and Process

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #11294 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 8034589 has failed:

On branch mergify/bp/2.24-maintenance/pr-11294
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 8034589d7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   tests/functional/lang/eval-fail-attrset-merge-drops-later-rec.err.exp
	new file:   tests/functional/lang/eval-fail-attrset-merge-drops-later-rec.nix
	new file:   tests/functional/lang/eval-okay-regrettable-rec-attrset-merge.exp
	new file:   tests/functional/lang/eval-okay-regrettable-rec-attrset-merge.nix
	modified:   tests/functional/lang/parse-fail-mixed-nested-attrs1.err.exp
	modified:   tests/functional/lang/parse-fail-mixed-nested-attrs2.err.exp
	modified:   tests/unit/libexpr/trivial.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libexpr/parser-state.hh

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally