swift234

Add a wrapper type for the union of ArrayRef and LazyMemberLoader+context.

This requires some careful handling since the base of an ArrayRef and the
lazy loading "context data" both have no free bits, but it now works.
(It's essentially a TwoPointerUnion type, just specialized for a particular
pair of two-pointer types.)

No functionality change, except that NominalTypeDecl and ExtensionDecl
are two words smaller.

Swift SVN r15910