<vector>: Test constexpr spaceships

[StephanTLavavej]

#1655 added constexpr to vector spaceship:

STL/stl/inc/vector

Lines 1909 to 1911 in 5547f0f

	template <class _Ty, class _Alloc>
	_NODISCARD _CONSTEXPR20_CONTAINER _Synth_three_way_result<_Ty> operator<=>(
	const vector<_Ty, _Alloc>& _Left, const vector<_Ty, _Alloc>& _Right) {

#1648 added vector::iterator spaceships with constexpr:

STL/stl/inc/vector

Line 156 in 5547f0f

_NODISCARD _CONSTEXPR20_CONTAINER strong_ordering operator<=>(const _Vector_const_iterator& _Right) const noexcept {

STL/stl/inc/vector

Line 2215 in 5547f0f

_NODISCARD _CONSTEXPR20_CONTAINER strong_ordering operator<=>(const _Vb_const_iterator& _Right) const noexcept {

When VS 2019 16.10 Preview 2 is available, we should test vector/vector<bool>'s container/iterator spaceships for constexpr (in tests/std/tests/P1614R2_spaceship/test.cpp).