P0722R3:  Implement library support for destroying delete

This provides the `std::destroying_delete_t` declaration in C++2a and after. (Even when the compiler doesn't support the language feature).

However, the feature test macro `__cpp_lib_destroying_delete` is only defined when we have both language support and  C++2a.

Reviewers: ldionne, ckennelly, serge-sans-paille, EricWF

Reviewed By: EricWF

Subscribers: dexonsmith, riccibruno, christof, jwakely, jdoerfert, mclow.lists, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D55840

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@361572 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed