<array>: array<const T, 0> allows fill() and swap()

Similar-but-distinct from #942; we should track it as a separate issue. The following libcxx tests are skipped:

https://github.com/microsoft/STL/blob/9e76d8c3d3700e086f3fbedd0c5b71f1678e4796/tests/libcxx/expected_results.txt#L543-L545
https://github.com/microsoft/STL/blob/9e76d8c3d3700e086f3fbedd0c5b71f1678e4796/tests/libcxx/skipped_tests.txt#L543-L545

Because they're implemented as no-ops:

https://github.com/microsoft/STL/blob/9e76d8c3d3700e086f3fbedd0c5b71f1678e4796/stl/inc/array#L600-L622

WG21-N4861 [array.members]/3 specifies fill() with "Effects: As if by fill_n(begin(), N, u)." and /4 specifies swap() with "Effects: Equivalent to swap_ranges(begin(), end(), y.begin())."

Notably, these "equivalent to" calls are ill-formed for const T (even if there happen to be zero of them), so libcxx is correct to expect these to fail to compile (this is what meow.fail.cpp means).

We should be able to fix this without breaking ABI; static_assert should be sufficient.

Start with the fill() and swap() implementations in stl/inc/array around the referenced lines, then inspect the corresponding libcxx entries in tests/libcxx/expected_results.txt and skipped_tests.txt. Run the relevant libcxx compile-fail tests; done means array<const T, 0> no longer accepts fill() or swap() while ABI remains unchanged.