swift234

Clang importer: introduce "alternate declarations" for redundant imported decls.

There are some Clang declarations that end up being imported as
multiple declarations in Swift, e.g., CF types get imported both with
and without the "Ref" suffix, subscript getters get imported both as a
method and as a subscript, etc. Track this explicitly so it's easy to
query the alternate declaration of a given declaration for name
lookup.

In many of these cases, the alternate declarations should simply go
away, because they're bloating the API. But that's a Swift 3 change
that requires review. For now, we want parity between the behavior
without and with Swift name lookup tables.