[clang] [HLSL][SPIR-V] Add hlsl_private address space for HLSL/SPIR-V (PR #122103)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Hi all!

This PR was dormant for a while because proposals had to be made & merged.
The HLSL proposal which covers `hlsl_private` is now merged: https://github.com/llvm/wg-hlsl/commit/4c9e11aa50727c53b10d232195fb88fd5376f19c

https://github.com/llvm/llvm-project/pull/122103