[libc++] Implement LWG 2510

Summary:
LWG2510 makes tag types like allocator_arg_t explicitly default
constructible instead of implicitly default constructible. It also
makes the constructors for std::pair and std::tuple conditionally
explicit based on the explicit-ness of the default constructibility
for the pair/tuple's elements.

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D65161

llvm-svn: 372777
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 95411dd426e6ea5b13c8f1bb7c4ba7190ecd6c1a
diff --git a/src/utility.cpp b/src/utility.cpp
index 016a5d9..6a690dc 100644
--- a/src/utility.cpp
+++ b/src/utility.cpp
@@ -10,6 +10,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-const piecewise_construct_t piecewise_construct = {};
+const piecewise_construct_t piecewise_construct{};
 
 _LIBCPP_END_NAMESPACE_STD