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

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

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.

Note: This patch is one of a series of patches to replace *all* std::sort to
llvm::sort.  Refer the comments section in D44363 for a list of all the
required patches.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329475 91177308-0d34-0410-b5e6-96231b3b80d8