[AST] Fix structural inequivalence of operators

Summary: Operators kind was not checked, so we reported e.g. op- to be equal with op+

Reviewers: shafik, a_sidorin, aaron.ballman

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57902

llvm-svn: 353504