Ryan Scott authored Oct 31, 2020 and Marge Bot committed Nov 02, 2020

Previously, the functions in `GHC.Core.Lint` used a patchwork of
different ways to display Core Lint errors:

* `lintPassResult` (which is the source of most Core Lint errors) renders
  Core Lint errors with a distinctive banner (e.g.,
  `*** Core Lint errors : in result of ... ***`) that sets them apart
  from ordinary GHC error messages.
* `lintAxioms`, in contrast, uses a completely different code path that
  displays Core Lint errors in a rather confusing manner. For example,
  the program in #18770 would give these results:

  ```
  Bug.hs:1:1: error:
      Bug.hs:12:1: warning:
          Non-*-like kind when *-like expected: RuntimeRep
          when checking the body of forall: 'TupleRep '[r]
          In the coercion axiom Bug.N:T :: []. Bug.T ~_R Any
          Substitution: [TCvSubst
                           In scope: InScope {r}
                           Type env: [axl :-> r]
                           Co env: []]
    |
  1 | {-# LANGUAGE DataKinds #-}
    | ^
  ```
* Further digging reveals that `GHC.IfaceToCore` displays Core Lint
  errors for iface unfoldings as though they were a GHC panic. See, for
  example, this excerpt from #17723:

  ```
  ghc: panic! (the 'impossible' happened)
    (GHC version 8.8.2 for x86_64-unknown-linux):
          Iface Lint failure
    In interface for Lib
    ...
  ```

This patch makes all of these code paths display Core Lint errors and
warnings consistently. I decided to adopt the conventions that
`lintPassResult` currently uses, as they appear to have been around the
longest (and look the best, in my subjective opinion). We now use the
`displayLintResult` function for all three scenarios mentioned above.
For example, here is what the Core Lint error for the program in #18770 looks
like after this patch:

```
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
*** Core Lint errors : in result of TcGblEnv axioms ***
Bug.hs:12:1: warning:
    Non-*-like kind when *-like expected: RuntimeRep
    when checking the body of forall: 'TupleRep '[r_axn]
    In the coercion axiom N:T :: []. T ~_R Any
    Substitution: [TCvSubst
                     In scope: InScope {r_axn}
                     Type env: [axn :-> r_axn]
                     Co env: []]
*** Offending Program ***
axiom N:T :: T = Any -- Defined at Bug.hs:12:1
*** End of Offense ***

<no location info>: error:
Compilation had errors
```

Fixes #18770.