Remove uses of deprecated std::pointer_to_unary_function

It's removed in C++17. No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313481

 91177308-0d34-0410-b5e6-96231b3b80d8
Signed-off-by: Guilherme Amadio <amadio@cern.ch>