Backport of update hcl v2.19.1 into v1.6

[ghost]

Backport

This PR is auto-generated from #34108 to be assessed for backporting due to the inclusion of the label 1.6-backport.

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

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Update hcl to v2.19.1, which fixes an issue where the decoder can return null or unknown values which still contain optional attribute type constraints.

Remove invalid configschema tests. Schema attributes must have a type to be valid. These would have panicked anyway when used in other cases, but were passing within the limited test case. The update to hcl makes these panic earlier, so add our own panics to prevent invalid attribute schemas entirely.

We can be fairly certain these panics didn't affect actual functionality outside of the synthetic tests, because hcldec was returning an invalid cty.NilVal with no error, which itself would have panicked later if used.

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Overview of commits

• 84f375a - 663cc1d - 6985a6c

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