[ADT] Add zip_longest iterators

Like the already existing zip_shortest/zip_first iterators, zip_longest
iterates over multiple iterators at once, but has as many iterations as
the longest sequence.

This means some iterators may reach the end before others do.
zip_longest uses llvm::Optional's None value to mark a
past-the-end value.

zip_longest is not reverse-iteratable because the tuples iterated over
would be different for different length sequences (IMHO for the same
reason neither zip_shortest nor zip_first should be reverse-iteratable;
one can still reverse the ranges individually if that's the expected
behavior).

In contrast to zip_shortest/zip_first, zip_longest tuples contain
rvalues instead of references. This is because llvm::Optional cannot
contain reference types and the value-initialized default does not have
a memory location a reference could point to.

The motivation for these iterators is to use C++ foreach to compare two
lists of ordered attributes in D48100 (SemaOverload.cpp and
ASTReaderDecl.cpp).

Idea by @hfinkel.

Differential Revision: https://reviews.llvm.org/D48348

llvm-svn: 348301