[clang-tidy] Add the abseil-time-subtraction check
Differential Revision: https://reviews.llvm.org/D58137
This patch introduced a build-failure for the documentation builders (see http://lab.llvm.org:8011/builders/clang-tools-sphinx-docs/builds/38188/steps/docs-clang-tools-html/logs/stdio).
Please take a look at the buildbots after you committed something (http://lab.llvm.org:8011/console) to avoid such issues.
This one is a small issue and I patched it.