[HLSL][SPIR-V] Add SV_GroupIndex semantic support

[sudonatalie]

Add SPIR-V backend support for the HLSL SV_GroupIndex attribute.

[Sh0g0-1758]

Hey @sudonatalie, I can take this up. Can you give me some pointers to get started.

[sudonatalie]

@Sh0g0-1758 You're more than welcome to give it a shot! Before getting started, you should make sure you can build clang with LLVM_EXPERIMENTAL_TARGETS_TO_BUILD="SPIRV" and LLVM_INCLUDE_SPIRV_TOOLS_TESTS=ON' and are able to run the check-llvm-codegen-spirv target tests.

The implementation should be similar to #82536, except that the concept of an SV_GroupIndex HLSL semantic translates to a LocalInvocationIndex SPIR-V builtin. I'd recommend taking a look at some of the following resources for background:

• User Guide for SPIR-V Target
• HLSL: SV_GroupIndex
• SPIR-V: LocalInvocationIndex