[SPIR-V] SV_Barycentrics doesn't work

[kh-huawei]

Description
SV_Barycentrics doesn't work with Spirv on vulkan devices.

There was an attempt to add support for the extenion KHR_fragment_shading_barycentric with PR 4638, which was later reverted with PR 5384, and since then nothing happened, leading to the issue that SV_Barycentrics doesn't work when using Vulkan with Nvidia devices.

Steps to Reproduce
Create a fragment shader that uses SV_Barycentrics

Actual Behavior
The Barycentric values are wrong

Environment

• DXC version 1.7.0.4010, but newer versions don't change anything there
• Host Operating System Windows 10

[s-perron]

@kh-huawei Can you please provide an sample that produces the wrong results? I thought the feature was reintroduced #5649.