Invalid SPIR-V generated from selection of __constant pointers

```
__constant int const_array[] = {0,1,2,3};

void bar(__global int* data, __constant int* c1, __constant int* c2, int x) {
__constant int* c = (x == 0) ? c1 : c2;
*data = *c;
}

__kernel void foo(__global int* data, __constant int* const_arg, int x) {
bar(data, const_array, const_arg, x);
}
```

clspv generates `const_array` as a variable in the `Private` storage class. The ternary statement in `bar` attempts to produce a `StorageBuffer` storage class pointer by selecting between a `Private` storage class pointer and `StorageBuffer` storage class pointer. This is invalid SPIR-V.

The resolution of this issue should also work with #208.