Fix applying sp_before_dc rule with decltype

Rule wasn't applied and one space was forcely added in the following example:
std::vector<uint8_t> v;
decltype(v)::const_iterator it;

It becomes:
std::vector<uint8_t> v;
decltype(v) ::const_iterator it;

I added decltype keyword by analogy with typeof. It would be better to create a separate rule for decltype/typeof in the future.