Fix array sizes where address space is not yet known

For variables in generic address spaces, for example:

```
unsigned char V[6442450944];
...
```

the address space is not yet known when we get into
*getConstantArrayType*, it is 0. AMDGCN target's
address space 0 has 32 bits pointers, so when we
call *getPointerWidth* with 0, the array size is
trimmed to 32 bits, which is not right.

Differential Revision: https://reviews.llvm.org/D30845

llvm-svn: 298420