<xmemory>: Investigate ASan annotation for overaligned "big" allocations

[CaseyCarter]

The implementation of std::allocator::allocate boosts the alignment of large allocations to help vectorization be more effective:

STL/stl/inc/xmemory

Lines 208 to 228 in faccf00

	#ifdef __cpp_aligned_new
	if constexpr (_Align > __STDCPP_DEFAULT_NEW_ALIGNMENT__) {
	size_t _Passed_align = _Align;
	#if defined(_M_IX86) || defined(_M_X64)
	if (_Bytes >= _Big_allocation_threshold) {
	// boost the alignment of big allocations to help autovectorization
	_Passed_align = (_STD max)(_Align, _Big_allocation_alignment);
	}
	#endif // defined(_M_IX86) || defined(_M_X64)
	return _Traits::_Allocate_aligned(_Bytes, _Passed_align);
	} else
	#endif // defined(__cpp_aligned_new)
	{
	#if defined(_M_IX86) || defined(_M_X64)
	if (_Bytes >= _Big_allocation_threshold) {
	// boost the alignment of big allocations to help autovectorization
	return _Allocate_manually_vector_aligned<_Traits>(_Bytes);
	}
	#endif // defined(_M_IX86) || defined(_M_X64)
	return _Traits::_Allocate(_Bytes);
	}

For allocations whose nominal alignment is at most __STDCPP_DEFAULT_NEW_ALIGNMENT__, we ask operator new for extra memory in order to perform the alignment manually:

STL/stl/inc/xmemory

Lines 152 to 169 in faccf00

	template <class _Traits>
	__declspec(allocator) void* _Allocate_manually_vector_aligned(const size_t _Bytes) {
	// allocate _Bytes manually aligned to at least _Big_allocation_alignment
	const size_t _Block_size = _Non_user_size + _Bytes;
	if (_Block_size <= _Bytes) {
	_Throw_bad_array_new_length(); // add overflow
	}
	const uintptr_t _Ptr_container = reinterpret_cast<uintptr_t>(_Traits::_Allocate(_Block_size));
	_STL_VERIFY(_Ptr_container != 0, "invalid argument"); // validate even in release since we're doing p[-1]
	void* const _Ptr = reinterpret_cast<void*>((_Ptr_container + _Non_user_size) & ~(_Big_allocation_alignment - 1));
	static_cast<uintptr_t*>(_Ptr)[-1] = _Ptr_container;
	#ifdef _DEBUG
	static_cast<uintptr_t*>(_Ptr)[-2] = _Big_allocation_sentinel;
	#endif // defined(_DEBUG)
	return _Ptr;
	}

It would be nice to enhance our ASan experience by poisoning the non-user space in these allocations to detect under/overflow. Doing so will probably require adding more symbols from sanitizer/asan_interface.h or sanitizer/common_interface.h to __msvc_sanitizer_annotate_container.hpp, as well as a new opt-out macro and detect_mismatch pragma.