Implement LWG-3869 Deprecate std::errc constants related to UNIX STREAMS

[frederick-vs-ja]

Fixes #3420.

It seems meaningful to apply the deprecation to older modes, since the changes in the upstream standards (original version of POSIX 2008) predated C++11.

[CaseyCarter]

It seems meaningful to apply the deprecation to older modes, since the changes in the upstream standards (original version of POSIX 2008) predated C++11.

I don't think there's any value to deprecating these in older language modes. Certainly not enough to have to justify doing so to that one customer with an unmaintained fifth-party dependency who has to figure out what to do when we break them.

[StephanTLavavej]

Thanks! FYI @CaseyCarter @strega-nil-ms , I pushed 3 commits: Oxford comma, fix message to mention the correct SILENCE macro, and rename the macros to talk about UNIX_STREAMS plural. Please meow if you have concerns.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for warning users away from these useless constants! ⚠️ 😸 🚀