Set the HLSL supported compiler version to 2018

[zlatinski]

Set the HLSL-supported compiler version to 2018

Fixes #6462

The compiler is already supporting the new HLSL 2021 features, such as:

Slang is already enforcing ternary operator behavior
Slang supports HLSL 2021 features like select()

However, SLANG does not support templates. So we are rolling back the version to 2018.

// Test file with HLSL 2021 compliant code

// Simple entry point
float4 main() : SV_Target0
{
    // HLSL 2021 compliant example
    float4 a = float4(1.0, 2.0, 3.0, 4.0);
    float3 b = float3(5.0, 6.0, 7.0);
    bool condition = true;
    
    // This is valid in HLSL 2021 - explicit conversion
    float4 result = condition ? a : float4(b, 1.0);
    
    return result;
}
// Test file using HLSL 2021 select() function

// Simple entry point
float4 main() : SV_Target0
{
    float2 v = float2(0.5, -0.3);
    
    // Using select() function (HLSL 2021 style)
    float2 result = (1.0 - abs(v.yx)) * select(v.xy >= 0.0, 1.0, -1.0);
    
    return float4(result, 0.0, 1.0);
}
// Test file to check HLSL 2021 conditional operator behavior

// Simple entry point
float4 main() : SV_Target0
{
    // Pre-HLSL 2021 behavior example
    float4 a = float4(1.0, 2.0, 3.0, 4.0);
    float3 b = float3(5.0, 6.0, 7.0);
    bool condition = true;
    
    // This would work in pre-2021 HLSL but fail in HLSL 2021
    // (implicit conversion from float3 to float4)
    float4 result = condition ? a : b;
    
    return result;
}

[zlatinski]

Fixes #6462

[jkwak-work]

Wouldn't this PR conflict with the PR 6445?

• Enable "-HV 2021" option for DXC #6445

I am going to submit 6445 this week; if not today.

[zlatinski]

Wouldn't this PR conflict with the PR 6445?

• Enable "-HV 2021" option for DXC #6445

I am going to submit 6445 this week; if not today.

Maybe? There was a big discussion about the HLSL version supported in SLANG.

csyonghe, do you want to comment on that?

[csyonghe]

These are two different things.

1. What should we #define HLSL_VERSION to the input HLSL code to Slang? -- Should be 2018.
2. What is the version of the HLSL code the Slang outputs? It is 2021, so we should pass -HV2021 when compiling the output HLSL code through DXC.

[jkwak-work]

Looks good to me.

Another PR of mine is not going to make it any time soon.
#6445