[unittests] Do not use llvm::sort in googlemock

Summary:
This reverts r329475 which applied to googlemock. This change makes the
googlemock implementation in LLVM dependent on LLVM unnecessarily.

Reviewers: echristo, mgrang

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52287

llvm-svn: 342612