Backport of issensitive must return unknown for unknown args without sensitive into v1.10

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Backport

This PR is auto-generated from #36012 to be assessed for backporting due to the inclusion of the label 1.10-backport.

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

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Terraform attempts to track marks as accurately as possible, but unknown values may not always have the same marks as they will when they become known. This is most easily seen with functions, which are allowed to return an unknown value when faced with any unknown arguments, while they are also allowed to manipulate the marks on the values as they see fit. This results in situations where the marks simply cannot be known.

Terraform generally takes the stance that if an unknown has a mark, it will remain in the final value, but the absence of a mark is not indicative of the absence of any marks in the final value. That appears to be something we can continue to maintain throughout the codebase, so given that axiom I'm going to codify it here by only changing the issensitive results for unknown, unmarked values, but allowing unknown+sensitive values to return true.

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Overview of commits

• b4c24c2

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Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.