Sema: Prefer an outer type named 'Self' over the SE-0068 behavior
This fixes a recent source break. We need to perform the normal
unqualified lookup before we handle the special case of 'Self',
because there might be a type named Self defined in an outer
context.
Fixes <https://bugs.swift.org/browse/SR-12133> / <rdar://problem/59216636>
