Make internal class __wrap_iter constexpr when not using libc++'s debugging mode. Introduce a new macro _LIBCPP_CONSTEXPR_IF_NODEBUG to mark this.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337019 91177308-0d34-0410-b5e6-96231b3b80d8

include/__config

@@ -1023,6 +1023,14 @@ template <unsigned> struct __static_assert_check {};
 #  define _LIBCPP_EXPLICIT_MOVE(x) (x)
 #endif
 
+#ifndef _LIBCPP_CONSTEXPR_IF_NODEBUG
+#if !defined(_LIBCPP_DEBUG) && _LIBCPP_STD_VER >= 14
+#define _LIBCPP_CONSTEXPR_IF_NODEBUG constexpr
+#else
+#define _LIBCPP_CONSTEXPR_IF_NODEBUG
+#endif
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ASAN
 _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);