Map FactoryAsInit: C to a hidden attribute.
The FactoryAsInit entry in API notes wasn't getting mapped to any
Clang attributes. Since there is no use for such an attribute purely
in Objective-C, map it to a new unspellable attribute
(SwiftSuppressFactoryAsInitAttr) used only to appropriately annotate
the declaration for consumption by the Swift Clang importer.
Part of rdar://problem/24447420.