Ensure for_each values wholly known for sets

[pselle]

In the case of a set that has some unknown values during the course of a plan (such as a set referencing a variable in an expression), Terraform would panic as it would attempt to convert the unknown value to a string in the evaluation of for_each. Instead, we should check for whollyknown state for for_each in the case of sets, so that we can carry on and the appropriate error will be displayed if a value is indeed unknown, or in the case of the bug, a variable will be evaluated and the plan/apply can continue.

Fixes #22580, #22629

[venky999]

Hi I am seeing the same issue ...is this going in next release and what would be the terraform version
@pselle @jbardin

[pselle]

@venky999 It'll be in the next release, and that will be 0.12.8.

[venky999]

@pselle thank you

[ghost]

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