[SPIR-V] SPV_EXT_descriptor_heap: Remove error diagnostic for discarded descriptor heap indexing`

Description

Discarded descriptor heap indexing (ResourceDescriptorHeap[i]; as a bare statement) is valid HLSL. The DXIL case which used to crash was fixed in #8749, which established the intended silent no-op behavior.

The current SPIR-V path instead emits an error:

error: ResourceDescriptorHeap/SamplerDescriptorHeap indexing must be used as a resource

This is now inconsistent with DXIL and incorrect by the language.

Fix

In doCXXOperatorCallExpr (SpirvEmitter.cpp), drop the emitError when parentMap->getParent(expr) yields no parent cast. Keep the null guard and return nullptr (result is unused). Remove or repurpose sm6_6.descriptorheap.ext.discarded.error.hlsl.

References

• #8749 DXIL fix + discussion establishing correct behavior
• https://github.com/microsoft/DirectXShaderCompiler/pull/8517#discussion_r3750881342) original review discussion flagged by @llvm-beanz

Start in SpirvEmitter.cpp at doCXXOperatorCallExpr and compare the discarded descriptor heap handling with the DXIL fix from #8749. Remove the diagnostic while preserving the null guard and unused-result return, then remove or repurpose sm6_6.descriptorheap.ext.discarded.error.hlsl and run the related SPIR-V tests to verify silent handling.