The previous commit added this flag but did not implement it. Here we
implement it by adjusting the shape of schema we return to Terraform Core
to mark the attribute as untyped and then ensure that gets handled
correctly on the SDK side.
This makes some slight adjustments to the shape of the schema we
present to Terraform Core without affecting how it is consumed by the
SDK and thus the provider. This mechanism is designed specifically to
avoid changing how the schema is interpreted by the SDK itself or by the
provider, so that prior behavior can be preserved in Terraform v0.11 mode.
This also includes a new rule that Computed-only (i.e. not also Optional)
schemas _always_ map to attributes, because that is a better mapping of
the intent: they are object values to be used in expressions. Nested
blocks conceptually represent nested objects that are in some sense
independent of what they are embedded in, and so they cannot themselves be
computed.
Martin Atkins