Remove XP (and Server 2003) support from STL

[AlexGuteniev]

Resolves #1192

[AlexGuteniev]

Actually probably need to keep not only _CRTIMP2, but every function visible from header

[AlexGuteniev]

I'll only remove functions in anonymous namespace, others cannot be safely removed.

[StephanTLavavej]

This looks good, thanks for working on this extensive set of changes! I'll push fixes for ABI compat and a few other things.

[StephanTLavavej]

Awesome - your PR shrinks the STL's main DLL by 1%! This is an extremely rare gift, thank you! 🎉

Changelog for the commits I pushed:

• In primitives.hpp, add static_asserts for the size/alignment of stl_MEOW_concrt, and consolidate the comments into a single TRANSITION comment.
• winapisupp.cpp no longer needs intrin.h; it was previously needed for __crtInitOnceExecuteOnce. (There will be a simple merge conflict with Cleanup headers #1270.)
• In winapisupp.cpp, SubmitThreadpoolWork and CloseThreadpoolWork are void functions, so we don't need to return them.
• Fix ABI: awint.hpp was declaring some functions as _CRTIMP2, which expands to __declspec(dllexport). To preserve the exported-ness of these functions, I've simply marked their definitions as _CRTIMP2 (there were a couple of existing occurrences for precedent). This avoids the verbosity of restoring the awint.hpp declarations.
• Comment all of the now-unused wrapper functions as "TRANSITION, ABI: preserved for binary compatibility", so they can be immediately eliminated in vNext.
• In filesystem.cpp, previously different preprocessor blocks became possible to fuse; I additionally adjusted their style to match the following preprocessor logic.
• In winapisupp.cpp, CreateSymbolicLinkW was previously being loaded dynamically, which was being changed to a direct call. However, it is documented as "desktop apps only", and both filesys.cpp and filesystem.cpp guard their calls with _CRT_APP. I'm 98% certain that winapisupp.cpp should do the same thing.

[StephanTLavavej]

I believe I've successfully ported your stl/CMakeLists.txt changes to stl/msbuild - I pushed a commit.

[StephanTLavavej]

Thanks again for this wonderful improvement - ripping out unnecessary complicated code is so awesome. 😸