[libcxx][iterator] adds `std::ranges::next`

[cjdb]

Summary

Implements part of P0896 'The One Ranges Proposal'.
Implements [range.iter.op.next].

Depends on D101922.

[ldionne]

You need to mark it [[nodiscard]] as an extension in the synopsis in <iterator>.

libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.next/next.pass.cpp

13 ↗

(On Diff #345658)

Normally, we have one test file per overload. I know this slipped through for most ranges reviews so far, but it would be good to do it.

67 ↗

(On Diff #345658)

Drop [[nodiscard]] from the tests please!

libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.next/next.verify.cpp

1 ↗

(On Diff #345658)

The nodiscard part should be in libcxx/test/libcxx since it's an extension.

[cjdb]

Applies @ldionne's feedback:

• removes [[nodiscard]] from ranges::next (per offline discussion: things not explicitly marked [[nodiscard]] in the wording to be on hold till after P2351R0 and P2377R0 are discussed in Library Evolution)
• removes [[nodiscard]] from tests
• splits single next.pass.cpp into four files, one for each overload

[cjdb]

rebases to see if CI started working properly