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

[Nathan Sidwell]

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.

-- 
Nathan Sidwell
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