swift234

[Name lookup] Don't allow unavailable-in-current-Swift declarations to shadow available ones.

The unavailable-in-current-Swift declarations introduced by the Clang
importer to help with migrating Swift 2.x code to Swift 3's naming
cause problems with unqualified name lookup when they shadow, e.g.,
types. The biggest problem in practice is with "URL", which is a
common Cocoa property name (in Swift 2) that becomes "url" in Swift 3,
but the old name conflicts with the Foundation value type "URL".

Fixes <rdar://problem/26236989>.