Fix nm on GCC 5.1 after the C++14 move

Summary:
As in D66306, fix the invocation of std::sort with std::function by not using
std::function, since it's easier to read and is broken in libstdc++ from GCC 5.1
(see https://gcc.gnu.org/PR65942).

Reviewers: thakis

Subscribers: jkorous, mgrang, dexonsmith, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66310

llvm-svn: 369045