An intermediate step in the Attributes rewrite.

Modify the AttrBuilder class to store the attributes as a set instead of as a
bit mask. The Attribute class will represent only one attribute instead of a
collection of attributes.

This is the wave of the future!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171427 91177308-0d34-0410-b5e6-96231b3b80d8