[OpenCL] always use SPIR address spaces for kernel_arg_addr_space MD

It doesn't make sense to use the target's address space ids in this context as
this is metadata that should be referring to the "logical" OpenCL address spaces.
For flat AS machines like all "CPUs" in general, the logical AS info gets lost as
there's only one address space (0).

This commit changes the logic such that we always use the SPIR address space
ids for the argument metadata. It thus allows implementing the clGetKernelArgInfo()
and the other detection needs.

https://reviews.llvm.org/D26157

llvm-svn: 286819