If a declaration provided in a Swift overlay has the same signature as
a declaration imported from the Clang module it overlays, or some
private framework that re-exports its interface through that Clang
module, the declaration provided in the Swift overlay shadows the one
imported from Clang.
To make this work for unqualified lookup, allow the shadowing rules to
kick in there as well. Make this a very narrow fix, because the
underlying problems that prevented us from doing this shadowing still
persist.
Fixes rdar://problem/35691030 and its several dupes.
