[polly] Change std::sort to llvm::sort in response to r327219

Summary:
r327219 added wrappers to std::sort which randomly shuffle the container before sorting.
This will help in uncovering non-determinism caused due to undefined sorting
order of objects having the same key.

To make use of that infrastructure we need to invoke llvm::sort instead of std::sort.

Reviewers: grosser, efriedma, jdoerfert, bollu, sebpop

Reviewed By: sebpop

Subscribers: sebpop, mehdi_amini, llvm-commits, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D44361

llvm-svn: 327361