Plug line_header leaks

Plug line_header leaks

This plugs a couple leaks introduced by commit fff8551cf549
("dwarf2read.c: Some C++fycation, use std::vector, std::unique_ptr").

The first problem is that nothing owns the temporary line_header that
handle_DW_AT_stmt_list creates in some cases.  Before the commit
mentioned above, the temporary line_header case used to have:

  make_cleanup (free_cu_line_header, cu);

and that cleanup was assumed to be run by process_die, after
handle_DW_AT_stmt_list returns and before child DIEs were processed.

The second problem is found in setup_type_unit_groups: that also used
to have a similar make_cleanup call, and ended up with a similar leak
after the commit mentioned above.

Fix both cases by recording in dwarf2_cu whether a line header is
owned by the cu/die, and have process_die explicitly free the
line_header if so, making use of a new RAII object that also replaces
the reset_die_in_process cleanup, while at it.

Thanks to Philippe Waroquiers for noticing the leak and pointing in
the right direction.

gdb/ChangeLog:
2017-08-17  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (struct dwarf2_cu) <line_header_die_owner>: New
	field.
	(reset_die_in_process): Delete, replaced by ...
	(process_die_scope): ... this new class.  Make it responsible for
	freeing cu->line_header too.
	(process_die): Use process_die_scope.
	(handle_DW_AT_stmt_list): Record the line header's owner CU/DIE in
	cu->line_header_die_owner.  Don't release the line header if it's
	owned by the CU.
	(setup_type_unit_groups): Make the CU/DIE own the line header.
	Don't release the line header here.