Turn an unmatched ]] back into two ]

Needed to handle e.g. `a[a[0]]`.  SWIG's parser doesn't seem to handle
that currently though.  See #2286.

Examples/test-suite/cpp11_attribute_specifiers.i

@@ -40,4 +40,11 @@ struct [[nodiscard]] S { };
 
 const char *test_string_literal() { return "Test [[ and ]] in string literal"; }
 
+#if 0
+// Check that SWIG doesn't choke on ]] when it's not part of an attribute.
+// FIXME: SWIG's parser doesn't handle this case currently.
+int *a;
+int b = a[a[0]];
+#endif
+
 %}