libstdc++: Use reserved form of [[__likely__]] in <variant>

We should not use [[unlikely]] before C++20, so use [[__unlikely__]]
instead.

libstdc++-v3/ChangeLog:

	* include/std/variant (_Variant_storage::_M_reset): Use
	__unlikely__ form of attribute instead of unlikely.

libstdc++-v3/include/std/variant

@@ -492,7 +492,7 @@ namespace __variant
       constexpr void
       _M_reset()
       {
-	if (!_M_valid()) [[unlikely]]
+	if (!_M_valid()) [[__unlikely__]]
 	  return;
 
 	std::__do_visit<void>([](auto&& __this_mem) mutable