Namely class methods, default signatures and pattern synonyms.

When scoped type variables occur inside class default methods,
default signatures and pattern synonyms, avoid re-create explicit
type variables when represent the type signatures.

This patch should fix Trac#14885.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

Test Plan: make test TEST="T14885a T14885b T14885c"

Reviewers: goldfire, bgamari, simonpj, RyanGlScott

Reviewed By: simonpj, RyanGlScott

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14885

Differential Revision: https://phabricator.haskell.org/D4469