libcpp: Fix up -fdirectives-only handling of // comments on last line not terminated...

libcpp: Fix up -fdirectives-only handling of // comments on last line not terminated with newline [PR100646]

As can be seen on the testcases, before the -fdirectives-only preprocessing
rewrite the preprocessor would assume // comments are terminated by the
end of file even when newline wasn't there, but now we error out.
The following patch restores the previous behavior.

2021-05-20  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/100646
	* lex.c (cpp_directive_only_process): Treat end of file as termination
	for !is_block comments.

	* gcc.dg/cpp/pr100646-1.c: New test.
	* gcc.dg/cpp/pr100646-2.c: New test.