forward_list.h (forward_list::assign): Dispatch to new functions based on assignabili...

forward_list.h (forward_list::assign): Dispatch to new functions based on assignability of elements.

	* include/bits/forward_list.h (forward_list::assign): Dispatch to new
	functions based on assignability of elements.
	(forward_list::_M_assign): Add overloaded functions for assigning
	via assignment or via clearing and insertion.
	(forward_list::_M_assign_val): Likewise.
	(forward_list::_M_move_assign(forward_list&&, false_type)): Do not
	erase elements that are not moved.
	* include/bits/forward_list.tcc (forward_list::operator=): Call
	assign() to copy elements.
	* testsuite/23_containers/forward_list/cons/10.cc: New.
	* testsuite/23_containers/forward_list/cons/11.cc: New.
	* testsuite/23_containers/forward_list/cons/12.cc: New.

From-SVN: r193057