libcpp: Fix up handling of block comments in -fdirectives-only mode [PR103130]

Normal preprocessing, -fdirectives-only preprocessing before the Nathan's
rewrite, and all other compilers I've tried on godbolt treat even \*/
as end of a block comment, but the new -fdirectives-only handling doesn't.

2021-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/103130
	* lex.c (cpp_directive_only_process): Treat even \*/ as end of block
	comment.

	* c-c++-common/cpp/dir-only-9.c: New test.

gcc/testsuite/c-c++-common/cpp/dir-only-9.c

@@ -0,0 +1,13 @@
+/* PR preprocessor/103130 */
+/* { dg-do preprocess } */
+/* { dg-options -fdirectives-only } */
+
+/*\
+ * this is a comment
+\*/
+
+int
+main ()
+{
+  return 0;
+}

libcpp/lex.c

@@ -4891,7 +4891,7 @@ cpp_directive_only_process (cpp_reader *pfile,
 			break;
 
 		      case '*':
-			if (pos > peek && !esc)
+			if (pos > peek)
 			  star = is_block;
 			esc = false;
 			break;