dwarf2dbg.c line_entry.next assert

dwarf2dbg.c line_entry.next assert

I was puzzling over how it was correct to cast what is clearly a
struct line_entry** pointer to a struct line_entry* pointer for a
few moments, and was going to write a comment but then decided we
really don't require the "next" pointer to be where it is.  Replace
the assert with an inline function that does any necessary pointer
adjustments.

	* dwarf2dbg.c (line_entry.next): Delete static assertion.
	(line_entry_at_tail): New inline function.
	(dwarf2_gen_line_info_1, dwarf2_finish): Replace casts in
	set_or_check_view arguments with line_entry_at_tail.