[clang-tidy] Fixed abseil-time-subtraction to work on C++17

Summary: Fixed abseil-time-subtraction to work on C++17

Reviewers: hokein, gribozavr

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63261

Patch by Johan Vikström.

llvm-svn: 363272