libstdc++: Simplify noexcept-specifiers for move constructors

This puts the logic for the noexcept-specifier in one place, and then
reuses it elsewhere. This means checking whether the move constructor
can throw doesn't need to do overload resolution and then check whether
some other constructor can throw, we just get the answer directly.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (_Hashtable::_S_nothrow_move()):
	New function to determine noexcept-specifier for move
	constructors.
	(_Hashtable): Use _S_nothrow_move() on move constructors.
	* testsuite/23_containers/unordered_map/cons/noexcept_move_construct.cc:
	Correct static assertion message.
	* testsuite/23_containers/unordered_multimap/cons/noexcept_move_construct.cc:
	Likewise.
	* testsuite/23_containers/unordered_multiset/cons/noexcept_move_construct.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/cons/noexcept_move_construct.cc:
	Likewise.