[SPIRV] Implement the llvm.spv.get.specialization.constant intrinsic

This intrinsic is used to get the value of a SPIR-V specialization constant.

The prototye is 

```
template <class T>
T llvm.spv.get.specialization.constant intrinsic(uint32_t ID, T DefaultValue);
```

where T is limited to bool, integer types, and floating point types.