Remove use of getImplicitlyUnwrappedOptionalDecl.
Remove use of getImplicitlyUnwrappedOptionalDecl.

As of https://github.com/apple/swift/pull/14299, we should effectively
no longer see this decl since we no longer generate IUO types.

A follow-on PR actually removes this API completely:
  https://github.com/apple/swift/pull/14364

(cherry picked from commit 9fac2e9efd609a0ea2adbe4b2782ffc35777b406)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb