LWG 2510, make the default constructors of library tag types explicit.

LWG 2510, make the default constructors of library tag types explicit.

2015-11-10  Ville Voutilainen  <ville.voutilainen@gmail.com>

	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.

From-SVN: r230175