Avoid parse errors for C++ attributes

Just ignore anything in between [[ and ]] in the scanner, which is better
that failing with a parse error.

Fixes #1158
Fixes #2286

Examples/test-suite/cpp11_attribute_specifiers.i

@@ -0,0 +1,41 @@
+%module cpp11_attribute_specifiers
+
+%inline %{
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations" // We're using a deprecated attribute here...
+#pragma GCC diagnostic ignored "-Wattributes"              // likely is C++20
+#pragma GCC diagnostic ignored "-Wunused-variable"         // We are using an unused variable on purpose here
+#pragma GCC diagnostic ignored "-Wunused-parameter"        // We are using an unused param on purpose here
+#endif
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#pragma clang diagnostic ignored "-Wattributes"
+#pragma clang diagnostic ignored "-Wunused-variable"
+#pragma clang diagnostic ignored "-Wunused-parameter"
+#endif
+
+
+[[noreturn]] void noReturn() { throw; }
+[[nodiscard]] bool noDiscard() { return true; }
+[[nodiscard, deprecated("This has been replaced")]] bool noDiscardDeprecated() { return true; }
+void maybeUnused1([[maybe_unused]] bool b) { }
+bool maybeUnused2(bool a, [[maybe_unused]] bool b) { return a; }
+
+[[deprecated, nodiscard]] bool likely([[maybe_unused]] bool a, bool b) {
+  [[maybe_unused]] bool c = b;
+  if (b) [[likely]] {
+    return true;
+  } else [[unlikely]] {
+    if(a) {
+      return true;
+    }
+  }
+  return false;
+}
+
+struct [[nodiscard]] S { };
+
+%}