Fix tyvar scoping within class SPECIALISE pragmas

Type variables from class/instance headers scope over class/instance
method type signatures, but DO NOT scope over the type signatures in
SPECIALISE and SPECIALISE instance pragmas.

The logic in GHC.Rename.Bind.rnMethodBinds correctly accounted for
SPECIALISE inline pragmas, but forgot to apply the same treatment
to method SPECIALISE pragmas, which lead to a Core Lint failure with
an out-of-scope type variable. This patch makes sure we apply the same
logic for both cases.

Fixes #22913

(cherry picked from commit 9ee761bf)