[clang] [llvm] [SPIR-V] Fixup storage class for global private (PR #116636)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Hello all!
Changed the PR to use a new AS emitted by the FE. This way there are no more weird storage class switch.
However, this required to support a new `addrspacecast` operation from `Function` to `Private` and the other way around.

The FE change will be in another PR (can be merged later on)

https://github.com/llvm/llvm-project/pull/116636