If the allocator isn't default constructible then checking if the
default constructor throws in an exception specification makes the
declaration invalid. Use the type trait instead.

	PR libstdc++/87809
	* include/bits/forward_list.h (_Fwd_list_impl::_Fwd_list_impl()): Use
	trait in exception-specification instead of possibly invalid
	expression.
	* include/bits/stl_bvector.h (_Bvector_impl::_Bvector_impl()):
	Likewise.
	* include/bits/stl_list.h (_List_impl::_List_impl()): Likewise.
	* include/bits/stl_vector.h (_Vector_impl::_Vector_impl()): Likewise.
	* testsuite/23_containers/forward_list/cons/87809.cc: New test.
	* testsuite/23_containers/list/cons/87809.cc: New test.
	* testsuite/23_containers/vector/bool/cons/87809.cc: New test.
	* testsuite/23_containers/vector/cons/87809.cc: New test.

From-SVN: r265626