cppfiles.c: Update comments.

cppfiles.c: Update comments.

        * cppfiles.c: Update comments.
	(destroy_include_file_node): Rename destroy_node.
	(find_or_create_entry): New function.
	(open_file, _cpp_fake_include): Use it.
	(handle_missing_header): New function, broken out of
	_cpp_execute include.  Don't segfault if there is no
	system or quoted path.
	(_cpp_execute_include): Use handle_missing_header.

From-SVN: r40423