swift234

[Sema] Allow "redeclarations" within a module if both declarations are private.

While we could allow declarations with the same name and type if all but one
are private, it feels a bit subtle that one declaration at top-level scope can
shadow another declaration at top-level scope elsewhere in the module. Let's
start with this for now.

Currently guarded by -enable-private-discriminators.

Part of rdar://problem/17632175

Swift SVN r21602