[ADT] Enable set_difference() to be used on StringSet

Summary: Re-land r362766 after it was reverted in r362823.

Reviewers: jhenderson, dsanders, aaron.ballman, MatzeB, lhames, dblaikie

Reviewed By: dblaikie

Subscribers: smeenai, mgrang, mgorny, dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62369

llvm-svn: 362835