[IR] Teach `llvm::User` to co-allocate a descriptor.

Summary:
With this change, subclasses of `llvm::User` will be able to co-allocate
a variable number of bytes (called a "descriptor") with the `llvm::User`
instance.  The co-allocated descriptor can later be accessed using
`llvm::User::getDescriptor`.  This will be used in later changes to
implement operand bundles.

This change steals one bit from `NumUserOperands`, but given that it is
still 28 bits wide I don't think this will be a practical issue.

This change does not allow allocating hung off uses with descriptors.
This only for simplicity, not for any fundamental reason; and we can
easily add this functionality later if needed.

Reviewers: reames, chandlerc, dexonsmith, kmod, majnemer, pete, JosephTremoulet

Subscribers: pete, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D12455

llvm-svn: 248453