Make one of the AttributeSet ctors maintain the invariant that the
attribute list is ordered by index.

Differential Revision: http://llvm-reviews.chandlerc.com/D1265

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187682 91177308-0d34-0410-b5e6-96231b3b80d8