Fixed google-readability-casting test to work in c++17

Summary: Fixed google-readability-casting.cpp to get tests working in c++17

Reviewers: gribozavr, hokein

Reviewed By: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63128

Patch by Shaurya Gupta.

llvm-svn: 363047