Implement mapping:
- `global`: 1
- `workgroup`: 3
- `private`: 0
Add `addressSpaceToStorageClass`, mapping GPU address spaces to SPIR-V
storage classes to be able to use SPIR-V's
`storageClassToAddressSpace`, mapping SPIR-V storage classes to LLVM
address spaces according to our mapping above *by definition*.
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Signed-off-by: Victor Perez <victor.perez@codeplay.com>
