Update test for bsf private helper

[AlexGuteniev]

More coverage for #2330

[StephanTLavavej]

I've pushed changes, now affecting product code, to fix /clr:pure test failures internally. The issue is that because the test was being run for all Standard modes (including /clr:pure), _Countr_zero_bsf was being directly tested, even though it's used for C++20 only (after the logic was fixed for C++17 constexpr gcd()). Changelog:

• <limits>
  • Centralize _HAS_TZCNT_BSF_INTRINSICS. Same architecture guard as before, but additionally block /clr:pure, CUDA, and ICC. (This makes it equivalent to _HAS_POPCNT_INTRINSICS but I didn't want to unify them here.)
  • Guard the logic with _HAS_TZCNT_BSF_INTRINSICS. __isa_available needs to be available for popcnt below, so don't guard it any more restrictively.
  • Additionally guard the tzcnt/bsf logic with _HAS_CXX20, since that's the only mode it's used in (no reason to define it and never use it).
  • Extract the _TZCNT_U32/_TZCNT_U64 macro definitions from the extern "C" block (they don't need to be within it) so they can be guarded by _HAS_CXX20.
  • Immediately after _Countr_zero_tzcnt, undef those macros, instead of doing it later.
  • In _Countr_zero, change the guard to _HAS_CXX20 && _HAS_TZCNT_BSF_INTRINSICS; we need the former for is_constant_evaluated and the latter for _Checked_x86_x64_countr_zero.
  • #undef _HAS_TZCNT_BSF_INTRINSICS at the end of the file.
• env.lst
  • Go back to the usual_20_matrix.lst.
• test.cpp
  • Drop _HAS_CXX20 guards, as we're always in C++20 mode.

[StephanTLavavej]

Thanks for improving this test coverage! 🧪 ✅ 🚀