[ADT] Clean up zip iterators. NFC.

*  Use inheriting constructors declarations to avoid introducing the
   `Base` typedef and duplicate constructor definitions. This should make
   things cleaner, especially since `zip_common` also exposes a `Base`
   typedef.
*  Drop unnecessary template parameters.
*  Avoid double negation in `zip_shortest`'s `operator==` and rename the
   comparison function for better readability.

Reviewed By: zero9178

Differential Revision: https://reviews.llvm.org/D145332