[IR] Add AttributeSet to hide AttributeSetNode* again, NFC

Summary:
For now, it just wraps AttributeSetNode*. Eventually, it will hold
AvailableAttrs as an inline bitset, and adding and removing enum
attributes will be super cheap.

This sinks AttributeSetNode back down to lib/IR/AttributeImpl.h.

Reviewers: pete, chandlerc

Subscribers: llvm-commits, jfb

Differential Revision: https://reviews.llvm.org/D31940

llvm-svn: 300014