[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.creation/to_array.pass.cpp

@@ -23,7 +23,8 @@
 #include "test_macros.h"
 #include "MoveOnly.h"
 
-int main(int, char**) {
+constexpr bool tests()
+{
   //  Test deduced type.
   {
     auto arr = std::to_array({1, 2, 3});
@@ -110,13 +111,12 @@ int main(int, char**) {
     assert(arr[0].b == .1);
   }
 
-  // Test constexpr.
-  {
-    constexpr std::array<int, 3> arr = std::to_array({1, 2, 3});
-    static_assert(arr[0] == 1);
-    static_assert(arr[1] == 2);
-    static_assert(arr[2] == 3);
-  }
+  return true;
+}
 
+int main(int, char**)
+{
+  tests();
+  static_assert(tests(), "");
   return 0;
 }