[Diagnostics] Diagnose misused xor as pow

Summary:
Motivation:
https://twitter.com/jfbastien/status/1139298419988549632
https://twitter.com/mikemx7f/status/1139335901790625793
https://codesearch.isocpp.org/cgi-bin/cgi_ppsearch?q=10+%5E&search=Search

Reviewers: jfb, rsmith, regehr, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: lebedev.ri, Quuxplusone, erik.pilkington, riccibruno, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63423

llvm-svn: 369217