Helpful "not in scope before the splice" went missing in 9.0

In the program below, `c` is not in scope where `a` is defined because of the intervening TH splice. The error message in 8.10 was helpful

```
test18.hs:6:5: error:
• Variable not in scope: c :: Int
• ‘c’ (line 11) is not in scope before the splice on line 8
```

In 9.0 the error message became unhelpful

```
test18.hs:6:5: error: Variable not in scope: c :: Int
```

----

```haskell
{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH.Syntax

a :: Int
a = c

$(pure [])

c :: Int
c = 0
```

Reproduce the diagnostic with the supplied test18.hs Template Haskell example and compare the 8.10 and 9.0 outputs. Trace the handling of the out-of-scope reference across the intervening splice; done means the 9.0 diagnostic again explains that c is not in scope before the splice on line 8.

Written by the indexing model from the issue text.