return an error with an invalid resource index

[jbardin]

Invalid indexed references to other resources were silently returning
unknown values when the resource didn't exist. These were only caught by
extra validation in data sources when the config still contained unknown
values during ReadDataSource.

Also add a check for unknown values when applying managed resources to
prevent sending unknown values.

Have GetResourceInstance return full diagnostics pointing to the invalid
reference in the config to help users troubleshoot the issue. This is
currently only done during Apply, as there is some uncertainly about
all instances being resolved in time for plan evaluation. This will
still catch errors for both resources and data sources and report
diagnostic errors, but unfortunately that won't happen until the apply
phase.

The original output for a datasource with this error would only be

configuration for data.null_data_source.foo[0].id still contains unknown values during apply (this is a bug in Terraform; please report it!)

And now will report

Error: Invalid resource index

  on main.tf line 7, in data "null_data_source" "bar":
   7:     a = data.null_data_source.foo[0].id

Resource data.null_data_source.foo has no index [0].

Fixes #22424

[jbardin]

Superseded by #22846 which fixes the evaluation as a whole, since terraform can't determine if the index evaluation will be needed at all (which is probably why it had to silently return unknown in the first place)

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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