re PR middle-end/56461 (GCC is leaking lots of memory)

	PR middle-end/56461
	* internal.h (struct cpp_buffer): Add to_free field.
	(_cpp_pop_file_buffer): Add third argument.
	* files.c (_cpp_stack_file): Set buffer->to_free.
	(_cpp_pop_file_buffer): Add to_free argument.  Free to_free
	if non-NULL, and if equal to file->buffer_start, also clear
	file->buffer{,_start,_valid}.
	* directives.c (_cpp_pop_buffer): Pass buffer->to_free
	to _cpp_pop_file_buffer.

From-SVN: r196497