apidiff: add more clear message about missing type

Change-Id: I4dec1fc68007695a85696229a7dd061a252a4db1
Reviewed-on: https://go-review.googlesource.com/c/exp/+/410854
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jean de Klerk <deklerk@google.com>

Author: jeanbza

apidiff/correspondence.go

@@ -1,6 +1,7 @@
 package apidiff
 
 import (
+	"fmt"
 	"go/types"
 	"sort"
 )
@@ -126,7 +127,7 @@ func (d *differ) corr(old, new types.Type, p *ifacePair) bool {
 		}
 
 	default:
-		panic("unknown type kind")
+		panic(fmt.Sprintf("unknown type kind %T", old))
 	}
 	return false
 }