[libc++] Support Apple Clang 10, which lacks constexpr dtor support.

[• Quuxplusone]

Summary

Merely building libc++.a shouldn't require a bleeding-edge compiler;
but in order to build libc++.a you need to compile src/optional.cpp,
which requires being able to parse include/optional (and for some reason
to do this in full C++2a mode). Apple Clang 10 (my current Xcode version)
doesn't do constexpr destructors (wg21.link/p0784), so it can't build
libc++.a for this solitary reason. Let's just fix this one thing.

[ldionne]

Merely building libc++.a shouldn't require a bleeding-edge compiler;

You can and should be using the runtimes build if you don't want to install a recent "system" compiler.

Let's just fix this one thing.

This is a matter of policy. It's easy to add small workarounds, but the sum of all those workarounds is what becomes harmful. We don't test on AppleClang 10 (nor on any Clang that old), so it's not supported. Is there a reason why you can't use a more recent AppleClang?

See https://lists.llvm.org/pipermail/llvm-dev/2021-March/149249.html for our compiler policy. I've been meaning to update the docs to explain that policy for a while now, I'll prioritize that now.

libcxx/include/__config

 #endif
 
 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX14_CONSTEXPR)
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17 constexpr
 #else
 #  define _LIBCPP_CONSTEXPR_AFTER_CXX17
 #endif
 
+#if _LIBCPP_STD_VER > 17 && defined(__cpp_constexpr_dynamic_alloc) && (__cpp_constexpr_dynamic_alloc >= 201907L)
+#  define _LIBCPP_CONSTEXPR_DESTRUCTOR constexpr
+#else
+#  define _LIBCPP_CONSTEXPR_DESTRUCTOR
+#endif
+
 // The _LIBCPP_NODISCARD_ATTRIBUTE should only be used to define other
 // NODISCARD macros to the correct attribute.
 #if __has_cpp_attribute(nodiscard) || defined(_LIBCPP_COMPILER_MSVC)
 #  define _LIBCPP_NODISCARD_ATTRIBUTE [[nodiscard]]
 #elif defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(_LIBCPP_CXX03_LANG)
 #  define _LIBCPP_NODISCARD_ATTRIBUTE [[clang::warn_unused_result]]
 #else
 // We can't use GCC's [[gnu::warn_unused_result]] and

libcxx/include/optional

struct __optional_destruct_base<_Tp, false>
     union
     {
         char __null_state_;
         value_type __val_;
     };
     bool __engaged_;
 
     _LIBCPP_INLINE_VISIBILITY
-    _LIBCPP_CONSTEXPR_AFTER_CXX17 ~__optional_destruct_base()
+    _LIBCPP_CONSTEXPR_DESTRUCTOR ~__optional_destruct_base()
     {
         if (__engaged_)
             __val_.~value_type();
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr __optional_destruct_base() noexcept
         :  __null_state_(),