libcpp: Fix up -fdirectives-only preprocessing of includes not ending with newline...

libcpp: Fix up -fdirectives-only preprocessing of includes not ending with newline [PR100392]

If a header doesn't end with a new-line, with -fdirectives-only we right now
preprocess it as
int i = 1;# 2 "pr100392.c" 2
i.e. the line directive isn't on the next line, which means we fail to parse
it when compiling.

GCC 10 and earlier libcpp/directives-only.c had for this:
  if (!pfile->state.skipping && cur != base)
    {
      /* If the file was not newline terminated, add rlimit, which is
         guaranteed to point to a newline, to the end of our range.  */
      if (cur[-1] != '\n')
        {
          cur++;
          CPP_INCREMENT_LINE (pfile, 0);
          lines++;
        }

      cb->print_lines (lines, base, cur - base);
    }
and we have the assertion
      /* Files always end in a newline or carriage return.  We rely on this for
         character peeking safety.  */
      gcc_assert (buffer->rlimit[0] == '\n' || buffer->rlimit[0] == '\r');
So, this patch just does readd the more less same thing, so that we emit
a newline after the inline even when it wasn't there before.

2021-05-12  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/100392
	* lex.c (cpp_directive_only_process): If buffer doesn't end with '\n',
	add buffer->rlimit[0] character to the printed range and
	CPP_INCREMENT_LINE and increment line_count.

	* gcc.dg/cpp/pr100392.c: New test.
	* gcc.dg/cpp/pr100392.h: New file.