[libcxx] Make std::tuple<> trivially constructible

Summary:
This is not mandated by the Standard, but it's nonetheless a nice
property to have, especially since it's so easy to implement. It
also shrinks our bug list!

PR41714

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D62618

llvm-svn: 363075
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 405570dc7a95cdf43cdee14b8a9f9c1657a6ad13
diff --git a/include/tuple b/include/tuple
index 7437d5c..b283666 100644
--- a/include/tuple
+++ b/include/tuple
@@ -907,7 +907,7 @@
 {
 public:
     _LIBCPP_INLINE_VISIBILITY
-    _LIBCPP_CONSTEXPR tuple() _NOEXCEPT {}
+    _LIBCPP_CONSTEXPR tuple() _NOEXCEPT = default;
     template <class _Alloc>
     _LIBCPP_INLINE_VISIBILITY
         tuple(allocator_arg_t, const _Alloc&) _NOEXCEPT {}