[libc++] Implement LWG 2510

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372777 91177308-0d34-0410-b5e6-96231b3b80d8
24 files changed