GHCi: Don't remove shadowed bindings from typechecker scope.

The shadowed out bindings are accessible via qualified names like
Ghci1.foo.  Since they are accessable in the renamer the typechecker
should be able to see them too.  As a consequence they show up in :show
bindings.

This fixes T11547

Test Plan:
Fixed current tests to accomodate to new stuff in :show bindings
Added a test that verifies that the typechecker doesn't crash

Reviewers: austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D2447

GHC Trac Issues: #11547