schema: Computed Value Hints for Downstream Validation

Consider the following example:

``` hcl
resource "aws_cloudtrail" "foobar" {
name = "tf-trail-foobar"
s3_bucket_name = "${aws_s3_bucket.foo.arn}" # ARN instead of name
s3_key_prefix = "/prefix"
include_global_service_events = false
}

resource "aws_s3_bucket" "foo" {
bucket = "tf-yada-test-trail"
force_destroy = true
policy = <

Start by reading the schema handling for Computed fields and the downstream validation path described in the issue. Use the aws_cloudtrail and aws_s3_bucket example to understand the failure, then define what an example value and its validation output must establish; the issue names no files or tests, so completion criteria need to be settled first.

Written by the indexing model from the issue text.