[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
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