Fix duplicated ConstantVector and add several LLVM classes

* Add _downcast_ for UndefValue
* Add getDouble/getFloat for ConstantFP to get the double/float value. Since there is no internal support for long double in python, also ignore it
* Add isCString/isString/getAsString/getAsCString for ConstantDataSequential
* Add ConstantDataVector/ConstantPointerNull/ConstantAggregateZero