[clang] [llvm] [HLSL][SPIR-V] Implements SV_Position for VS/PS I/O (PR #168735)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

@s-perron the diagnostic for the SV_Position used as output for the pixel shader is not that great because it reuses the current message. The next PR (https://github.com/llvm/llvm-project/pull/168743) adds a new method to diagnose it which improves the message

https://github.com/llvm/llvm-project/pull/168735