[SPIR-V] Semantic index limits for system values are not enforced on -spirv path

Description

The SPIR-V backend does not enforce system-value semantic index limits, so -spirv silently accepts semantics that the DXIL path rejects. It's just a missing diagnostic rather than bad codegen; the index is discarded and the emitted module becomes the legal index-0 spelling.

Steps to Reproduce

// repro.hlsl
[shader("vertex")]
float4 main(uint vid : SV_VertexID) : SV_Position1 {
    return float4((float)vid, 0.0, 0.0, 1.0);
}

dxc -T vs_6_8 -E main repro.hlsl -Fo repro.dxo          # rejected
dxc -T vs_6_8 -E main -spirv repro.hlsl -Fo repro.spv   # accepted

Actual Behavior

DXIL path errors as expected:

error: SV_Position semantic index exceeds maximum (0).
Validation failed.

SPIR-V path exits 0 with no diagnostic and emits the index-0 builtin, so the
1 is silently dropped:

OpDecorate %gl_Position BuiltIn Position

Other SVs behave the same way (SV_VertexID1, SV_Depth1, SV_DispatchThreadID1, SV_PrimitiveID1, ...). The once exception is SV_Target, since its bound is checked in the frontend rather than backend.

Start with repro.hlsl and run the two dxc commands to compare the DXIL and -spirv paths. Trace the SPIR-V backend's handling of system-value semantic indices, then verify that invalid indices produce a diagnostic instead of being silently emitted as index 0, including the listed SV examples while preserving the SV_Target exception.