libstdc++: Add assertions to std::mask_array operations [PR62196]

libstdc++: Add assertions to std::mask_array operations [PR62196]

Add assertions to diagnose incorrect uses of valarray masks.

The assignment operators of std::mask_array do not have any explicit
preconditions in the standard, but the assignment operator
valarray<T>::operator=(const mask_array<T>&) requires the lengths to
match, so it seems consistent to also require that when the operands are
reversed.  In support of that interpretation, libstdc++ has undefined
behaviour if the right-hand operand has more elements than are selected
by the mask, and libc++ has undefined behaviour if it has fewer
elements. Our std::mask_array stores the number of selected elements as
_M_sz so it's easy to add an assertion that checks it.

For the valarray::operator[] that takes a valarray<bool> mask,
[valarray.sub] in the standard says: "In each case the selected
element(s) shall exist." This makes it undefined to have a mask that
refers to out-of-range elements. We can easily check this too.

libstdc++-v3/ChangeLog:

	PR libstdc++/62196
	* include/bits/mask_array.h (mask_array): Add assertions to
	assignment operators.
	* include/std/valarray (valarray::operator[](valarray<bool>)):
	Add assertions.
	* testsuite/26_numerics/valarray/mask-1_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-2_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-3_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-4_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-5_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-6_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-7_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask-8_neg.cc: New test.
	* testsuite/26_numerics/valarray/mask.cc: New test.