[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
Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 77b9abfc8e89ca627e4f9a1cc206bea131db6db1
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index e862840..c04a883 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -168,6 +168,8 @@
------------------------------------------------- -----------------
**C++ 2a**
-------------------------------------------------------------------
+ ``__cpp_lib_array_constexpr`` ``201811L``
+ ------------------------------------------------- -----------------
``__cpp_lib_atomic_ref`` *unimplemented*
------------------------------------------------- -----------------
``__cpp_lib_bind_front`` *unimplemented*