[clang-tidy] Recommit: Add the abseil-duration-comparison check
This check finds instances where Duration values are being converted to a numeric value in a comparison expression, and suggests that the conversion happen on the other side of the expression to a Duration. See documentation for examples.
This also shuffles some code around so that the new check may perform in sone step simplifications also caught by other checks.
Compilation is unbroken, because the hash-function is now directly
specified for std::unordered_map, as 'enum class' does not compile as
key (seamingly only on some compilers).
Patch by hwright.
Reviewers: aaron.ballman, JonasToth, alexfh, hokein
Reviewed By: JonasToth
Subscribers: sammccall, Eugene.Zelenko, xazax.hun, cfe-commits, mgorny
Differential Revision: https://reviews.llvm.org/D54737