C++20 finally incorporated SD-6 feature test macros

1. __cpp_xxx language macros (pre-defined)
2. __cpp_lib_xxx library macros (#define'd in <version>

That means Microsoft now needs to implement them, finally.

We've been requiring them in the SD-6 form for all post-C++11 features (conscious policy decision to leave MSVC users out). No problem for language macros, as they're all pre-defined. Problem are the library macros:

• __cpp_lib_optional only defined in <optional>
• ok, so wrap #include <optional> in #if __has_include(<optional>)
• but: <optional> #error's when #include'd in C++14 builds

This particular issue is behind us, and we can hope that compiler vendors have learned the lesson to either not report back true on __has_include for newer-than-requested headers or else no longer #error out, but simply not declare anything.

Nevertheless, the __has_include dance is very annoying, so <version> may end up being the first C++20 feature we require from all compilers.

Availability:

• libc++ ≥ LLVM 7 (what about AppleClang?)
• libstd++ ≥ GCC 9 (what about QNX?)
• MS STL ≥ main@ {2019-09-05}

  (what did this ship in?)

• RougeWave (unknown, does anyone still ship that)
• Dinkumware (unknown, does anyone still ship that)

Depending on how far away we are on the various platforms, we may require a <version>, or we write a q20version.h.