Two ParamAttrsVectors which differ by a permutation

Two ParamAttrsVectors which differ by a permutation
of their elements do not yield the same ParamAttrsList,
though they should.  On the other hand, everyone seems
to pass such vectors with elements ordered by increasing
index, so rather than sorting the elements simply assert
that the elements are ordered in this way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41845 91177308-0d34-0410-b5e6-96231b3b80d8