[v3 PATCH] LWG 2510, make the default constructors of library tag types explicit.

[Jonathan Wakely]

On 10/11/15 22:01 +0200, Ville Voutilainen wrote:
>    LWG 2510, make the default constructors of library tag types
>    explicit.
>    * include/bits/mutex.h (defer_lock_t, try_lock_t,
>    adopt_lock_t): Add an explicit default constructor.
>    * include/bits/stl_pair.h (piecewise_construct_t): Likewise.
>    * include/bits/uses_allocator.h (allocator_arg_t): Likewise.
>    * libsupc++/new (nothrow_t): Likewise.
>    * testsuite/17_intro/tag_type_explicit_ctor.cc: New.

OK for trunk, thanks.