Backport of Account for NestedType in static traversal validation into v1.2

[teamterraform]

Backport

This PR is auto-generated from #31532 to be assessed for backporting due to the inclusion of the label 1.2-backport.

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

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There were a couple locations left which assumed that a schema Attribute type was only defined by the Type field, which would end up using an invalid cty.NilType when NestedType was defined. Add ImpliedType() to the Attribute type which checks for both Type and NestedType and use it in place of direct Type field references.

Leave the Object.ImpliedType method for now, since objchange does specifically split out the NestedType for validation of optional attributes.

Fixes #31527

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