c++: Add stringification testcase for CWG1709 [PR120778]

The CWG1709 just codifies existing GCC (and clang) behavior, so this
just adds a testcase for that.

2025-08-03  Jakub Jelinek  <[email protected]>

	PR preprocessor/120778
	* g++.dg/DRs/dr1709.C: New test.

Author: jakubjelinek

gcc/testsuite/g++.dg/DRs/dr1709.C

@@ -0,0 +1,18 @@
+// DR 1709 - Stringizing raw string literals containing newline
+// { dg-do run { target c++26 } }
+
+#define A(a) #a
+const char *a = A(a\f\\b"c");
+const char *b = A(R"abc(a\b
+
+)abc");
+
+int
+main ()
+{
+  if (a[1] != '\f' || a[2] != '\\' || a[4] != '"' || a[6] != '"')
+    __builtin_abort ();
+  if (b[1] != '"' || b[7] != '\\' || b[9] != '\n' || b[10] != '\n'
+      || b[11] != ')' || b[15] != '"')
+    __builtin_abort ();
+}