swift234

IRGen: Make our use of spare bits in heap objects tagged-pointer-safe.

Don't use spare bits on platforms that use ObjC tagged pointers when an enum payload involves a class-constrained existential, archetype, or ObjC-defined class type. If a payload is of a Swift-defined class type, we can still assume it's a real pointer and use its spare bits. Add an @unsafe_no_objc_tagged_pointer attribute that can be applied to protocols to denote that existentials bounded by that protocol can use spare bits; this is necessary to preserve the layout of bridged Array and Dictionary types, which should not be bound to tagged pointer types in practice (fingers crossed). Fixes <rdar://problem/16270219>.

Swift SVN r18781