PR libstdc++/87809 avoid invalid expressions in exception specifications

PR libstdc++/87809 avoid invalid expressions in exception specifications

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