[HLSL][SPIR-V] Handle SV_Position builtin in PS (#141759)
This commit is using the same mechanism as vk::ext_builtin_input to
implement the SV_Position semantic input.
The HLSL signature is not yet ready for DXIL, hence this commit only
implements the SPIR-V side.
This is incomplete as it doesn't allow the semantic on hull/domain and
other shaders, but it's a first step to validate the overall
input/output
semantic logic.
Fixes https://github.com/llvm/llvm-project/issues/136969
