ADL-proof implementation of algorithms in [alg.nonmodifying]

[frederick-vs-ja]

Towards #140 and #1596, separated from #4004.

The std::search overload taking a searcher is not yet handled. I'll created another PR for standard searchers.

• The modification of _Adl_verify_range is adopted from <algorithm>: unqualified calls to _Adl_verify_range incorrectly cause instantiation #1596 (comment) (thanks to @miscco).
• A single commit adding _STD, _RANGES, and ::-qualifications.
  • I decided to add :: for __std_meow functions because _CSTD __std_meow looks very weird to me.
• A single commit removing ADL-dependent structured binding declarations (where the unnamed variables are pairs).
  • I'm not sure whether we should avoid structured bindings or change the return type of _Equal_rev_pred to avoid ADL.
• A single commit speculatively implementing LWG-3969.
• A single commit for Clang-formatting.

Notes:

• Some _Ugly function names in <execution> are not yet qualified, because they take either no arguments or only integers.
• _Ubegin and _Uend are not qualified because they are function objects.

[StephanTLavavej]

Thanks for the fine-grained commits, this was very easy to review! I pushed a couple of fixes. I think it needs only one maintainer approval.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for adding defenses against our eternal nemesis, ADL! 🛡️ 📈 🎉