Backport of Improve marks support for length and lookup into v0.15

[teamterraform]

Backport

This PR is auto-generated from #28644 to be assessed for backporting due to the inclusion of the label 0.15-backport.

The below text is copied from the body of the original PR.

---

We improved marks support for these two functions in zclconf/go-cty#98, not realizing that Terraform has its own separate and incompatible implementations. These commits bring across essentially the same changes, re-applied to these local versions.

functions: Improve marks support for length

Similar to cty's implementation, we only need to preserve marks from the value itself, not any nested values it may contain. This means that taking the length of an umarked list with marked elements results in an unmarked number.

functions: Improve marks support for lookup

Several changes to lookup to improve how we handle marked values:

• If the entire collection is marked, preserve the marks on any result (whether successful or fallback)
• If a returned value from the collection is marked, preserve the marks from only that value, combined with any overall collection marks
• Retain marks on the fallback value when it is returned, combined with any overall collection marks
• Include marks on the key in the result, as otherwise the result it ends up selecting could imply what the sensitive value was
• Retain collection marks when returning an unknown value for a not wholly-known collection

Fixes #28638

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.