[SPIR-V] Using SV_Target and SV_Target0 in the same PS fails validation instead of printing an error

[Dredhog]

Title

Using SV_Target and SV_Target0 in the same PS fails validation instead of printing an error

Minimal repro steps

1. Compile the following pixel shader with dxc.exe -T ps_6_0 -E PSMain -spirv repro_shader.hlsl

struct PSInput
{
	float4 color : COLOR;
};

void PSMain(PSInput input,
            out float4 out_ : SV_Target,
            out float4 out_0 : SV_Target0,
            out float4 out_1 : SV_Target1)
{
    out_ = input.color;
    out_0 = input.color;
    out_1 = input.color;
}

Expected result

An error is printed e.g. like the one with the DXIL backend: "Function: PSMain: error: Parameter with semantic SV_Target has overlapping semantic index at 0. Use /Zi for source location." or similar.

Actual result

a validation error is printed, which asks to report a bug:

fatal error: generated SPIR-V is invalid: Entry-point has conflicting output location assignment at location 0, component 0
    OpEntryPoint Fragment %PSMain "PSMain" %in_var_COLOR %out_var_SV_Target %out_var_SV_Target0 %out_var_SV_Target1

note: please file a bug report on https://github.com/Microsoft/DirectXShaderCompiler/issues with source code if possible