c++: directives-only preprocessing and include translation [PR 99050]

We make sure files end in \n by placing one at the limit of the buffer
(just past the end of what is read).  We need to do the same for
buffers generated via include-translation.  Fortunately they have
space.

	libcpp/
	* files.c (_cpp_stack_file): Make buffers end in unread \n.
	gcc/testsuite/
	* g++.dg/modules/pr99050_a.H: New.
	* g++.dg/modules/pr99050_b.C: New.