Typechecker plugins aren't run when doing pattern-match checks
Very common code with type-level natural numbers:
```hs
{-# OPTIONS_GHC -fplugin GHC.TypeLits.Normalise #-}
data Vec :: Nat -> Type -> Type where
Nil :: Vec 0 a
(:>) :: a -> Vec n a -> Vec (n + 1) a
infixr 5 :>
merge :: Vec n a -> Vec n a -> Vec (n + n) a
merge Nil Nil = Nil
merge (x :> xs) (y :> ys) = x :> y :> merge xs ys
```
(or `zip`, `zipWith` etc).
This will, with `-Wincomplete-patterns`, lead to spurious "incomplete pattern match" errors.
What happens during pattern match checking is that we try to find whether the following Givens are inconsistent:
```
[G] n1 ~ 0
[G] n2 + 1 ~ n1
```
If we ran the usual type-checker plugins on this, we'd expect these Givens to be reported as inconsistent.
However, this wasn't happening. After a quick investigation, it seems that the `tcg_tc_plugin_solvers` field of `TcGblEnv` is empty in this context. Here's what I see in the `tc-trace`:
```
solveSimpleGivens {
[[G] pm_d10l_d10l {0} :: n1 ~# 0 (CNonCanonical)]
Emitting fresh work
{[G] pm_d10l_d10l {0} :: n1 ~# 0 (CNonCanonical)}
-----------------------------
Start solver pipeline {
tclevel = 0
work item = [G] pm_d10l_d10l {0} :: n1 ~# 0 (CNonCanonical)
inerts = {Type-function equalities = {[G] co_a10c {0}
:: n2 + 1 ~# n1}
Innermost given equalities = 0
Given eqs at this level = True}
rest of worklist = WL {}
[...]
End solver pipeline }
runTcPluginsGiven: no solvers -- trace I added in runTcPluginsGiven
End solveSimpleGivens }
```
So it looks to me that typechecker plugins are incorrectly initialised for calls within the pattern match checker.
issue