LWG 3035. std::allocator's constructors should be constexpr
	* include/bits/allocator.h (allocator): Add constexpr to constructors
	for C++2a. Replace dynamic exception specifications with NOTHROW
	macro.
	(allocator, operator==, operator!=): Replace USE_NOEXCEPT macro with
	NOTHROW.
	* include/bits/c++config (_GLIBCXX20_CONSTEXPR): Define.
	* include/ext/malloc_allocator.h (malloc_allocator): Add constexpr
	to constructors for C++2a.
	* include/ext/new_allocator.h (new_allocator): Likewise.

From-SVN: r261703