[libc++] Complete overhaul of constexpr support in std::array

This commit adds missing support for constexpr in std::array under all
standard modes up to and including C++20. It also transforms the <array>
tests to check for constexpr-friendliness under the right standard modes.

Fixes https://llvm.org/PR40124
Fixes rdar://57522096
Supersedes https://reviews.llvm.org/D60666

Differential Revision: https://reviews.llvm.org/D80452

libcxx/test/std/containers/sequences/array/array.tuple/tuple_element.fail.cpp

@@ -18,7 +18,6 @@
 #include <array>
 #include <cassert>
 
-
 // std::array is explicitly allowed to be initialized with A a = { init-list };.
 // Disable the missing braces warning for this reason.
 #include "disable_missing_braces_warning.h"