testsuite: Add test for already-fixed issue with _Pragma expansion [PR90400]

The PR was fixed by r12-5454. Since the fix was somewhat incidental,
although related, add a testcase from PR90400 too before closing it out.

gcc/testsuite/ChangeLog:

	PR preprocessor/90400
	* c-c++-common/cpp/pr90400.c: New test.

gcc/testsuite/c-c++-common/cpp/pr90400.c

@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-save-temps" } */
+/* PR preprocessor/90400 */
+
+#define OUTER(x) x
+#define FOR(x) _Pragma ("GCC unroll 0") for (x)
+void f ()
+{
+    /* If the pragma were to be seen prior to the expansion of FOR, as was
+       the case before r12-5454, then the unroll pragma would complain
+       because the immediately following statement would be ";" rather than
+       a loop.  */
+    OUTER (; FOR (int i = 0; i != 1; ++i);) /* { dg-bogus {statement expected before ';' token} } */
+}