Don't quantify implicit type variables when quoting type signatures in TH

Summary:
A bug was introduced in GHC 8.0 in which Template Haskell-quoted type
signatures would quantify _all_ their type variables, even the implicit ones.
This would cause splices like this:

```
$([d| idProxy :: forall proxy (a :: k). proxy a -> proxy a
      idProxy x = x
   |])
```

To splice back in something that was slightly different:

```
idProxy :: forall k proxy (a :: k). proxy a -> proxy a
idProxy x = x
```

Notice that the kind variable `k` is now explicitly quantified! What's worse,
this now requires the `TypeInType` extension to be enabled.

This changes the behavior of Template Haskell quoting to never explicitly
quantify type variables which are implicitly quantified in the source.

There are some other places where this behavior pops up too, including
class methods, type ascriptions, `SPECIALIZE` pragmas, foreign imports,
and pattern synonynms (#13018), so I fixed those too.

Fixes #13018 and #13123.

Test Plan: ./validate

Reviewers: simonpj, goldfire, austin, bgamari

Reviewed By: simonpj, goldfire

Subscribers: simonpj, mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2974

GHC Trac Issues: #13018, #13123

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/th/TH_pragma.stderr b/testsuite/tests/th/TH_pragma.stderr
index ddd5998b39..1156adee27 100644
--- a/testsuite/tests/th/TH_pragma.stderr
+++ b/testsuite/tests/th/TH_pragma.stderr
@@ -11,6 +11,6 @@ TH_pragma.hs:(10,4)-(12,31): Splicing declarations
         {-# SPECIALISE INLINE [~1] bar :: Float -> Float #-}
         bar x = x * 10 |]
   ======>
-    bar :: forall a. Num a => a -> a
+    bar :: Num a => a -> a
     {-# SPECIALISE INLINE [~1] bar :: Float -> Float #-}
     bar x = (x * 10)