Support the Standard Library Modules in C++20 mode

[StephanTLavavej]

Fixes #3945.

• stl/inc/ctime
  • Activate the workaround for internal OS-33790456 "static inline functions cannot be called when using modules" whenever _BUILD_STD_MODULE is defined, instead of additionally guarding it with _HAS_CXX23. This follows what <new> is already doing to detect whether we're building std.ixx:

    STL/stl/inc/new

    Line 20 in 60f1885

    #ifdef _BUILD_STD_MODULE

• stl/inc/yvals_core.h
  • Move the paper comment from the C++23 section to the C++20 section.
  • Originally, I silently ignored attempts to define _BUILD_STD_MODULE in too-old Standard modes. If they were then attempting to build std.ixx, they'd be confused by the results with no export appearing, and if they weren't building std.ixx then I was letting them get away with something very squirrely 🐿️. Now, _EXPORT_STD is simply directly controlled by _BUILD_STD_MODULE, but we have enforcement that immediately rejects attempts to do this in C++17 and older.
  • Move the feature-test macro from C++23 to C++20.
• stl/modules/std.ixx
  • Guard the 5 C++23-only headers (so far).
  • ⚠️ Stealth merge conflict with the remaining C++23 headers: <mdspan>, <flat_map>, <flat_set>, <generator>
• tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp
  • Update the feature-test macro test accordingly.
• tests/std/tests/P1502R1_standard_library_header_units/env.lst
  • Originally, I was testing the Standard Library Header Units (an official C++20 feature) only in /std:c++latest mode because it took a while to build, and we didn't have a lot of C++23 content yet. Now we should be more disciplined and actually test it in all possible Standard modes like everything else. Although Header Units emit macros, this test machinery is shared with the Named Modules test, so I'm having the test detect the Standard mode with a dedicated TEST_STANDARD macro (following a different convention than the product code, to avoid confusion).
• tests/std/tests/P2465R3_standard_library_modules/env.lst
  • Add C++20 coverage in an identical manner.
• tests/std/include/test_header_units_and_modules.hpp
  • Update the shared test code to guard the C++23-only headers.
• tests/std/tests/P1502R1_standard_library_header_units/test.cpp
  • Also guard the imports of C++23-only headers.

[CaseyCarter]

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

[CaseyCarter]

Thank you for bringing the slick modern magic of import std to those poor neanderthals stuck using the ancient C++20 standard.

[CaseyCarter]

Really late thought: will we have ODR problems in a project that includes both C++20 TUs and C++23 TUs that import std?

[StephanTLavavej]

Not any more than we already have with classic headers.