<bit> test coverage for /arch:AVX2

[Alex Guteniev (AlexGuteniev)]

Resolve #2149

[Stephan T. Lavavej (StephanTLavavej)]

• Verified that no other tests including <bit> would substantially benefit from these options.
• Verified that both tests are sensitive to /arch:AVX2 because bit_ceil() calls countl_zero().
• tests.py and features.py already know about /arch:AVX2 because floating_point_model_matrix.lst uses it.
• Accordingly, we don't have to worry about dev/test machine support (also double-checked the list of supported CPUs).
• popcount() is sensitive to /arch:AVX, but that's implied by /arch:AVX2, and I don't believe that adding /arch:AVX to the matrix would provide useful coverage:

  STL/stl/inc/limits

  Lines 1094 to 1103 in f75c7f5

  	#if _HAS_POPCNT_INTRINSICS
  	template <class _Ty>
  	_NODISCARD int _Checked_x86_x64_popcount(const _Ty _Val) noexcept {
  	constexpr int _Digits = numeric_limits<_Ty>::digits;
  	#ifndef __AVX__
  	const bool _Definitely_have_popcnt = __isa_available >= __ISA_AVAILABLE_SSE42;
  	if (!_Definitely_have_popcnt) {
  	return _Popcount_fallback(_Val);
  	}
  	#endif // !defined(__AVX__)

[Stephan T. Lavavej (StephanTLavavej)]

I'm mirroring this to an MSVC-internal PR. Changes can still be pushed during final review, but please notify me if that happens.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for adding this test coverage and preventing future regressions! 😻 🎉 ✔️