Return an error for invalid for expressions with marks

[pselle]

Rather than allowing a panic to occur, return an error for "Invalid object key" if a user attempts to write a for expression that would convert the marked value to an unmarked string (by using it as the object key).

Prior to this change, the following code would panic:

> local.baz
{
  "a" = (sensitive)
  "b" = "dog"
}
> {for  v in local.baz: v => 1 }

After:

> {for  v in local.baz: v => 1 }

>  
Error: Invalid object key

  on <console-input> line 1:
  (source code not available) # Terraform console usage

Marked values cannot be used as object keys.