[SPIR-V] allow users to access gl_InstanceIndex

[jaebaek]

This issue is from #3361 (comment)

Could the gl_InstanceIndex be exposed directly as a built in ([[vk::builtin("InstanceIndex")]])?
I'm using DXC to generate Vulkan shaders for PC where I want the functionality of gl_InstanceIndex with a non-zero BaseInstance. A built-in would allow SV_InstanceID to continue to work as is, while also allowing a shader access to the built-in SPIR-V BaseInstance and InstanceIndex variables. The SPIR-V DXC documentation is currently incorrect, because it states SV_InstanceID maps to the InstanceIndex built-in, with no mention of the "correction" applied to align it with DX12 semantics.

[sudonatalie]

Requests for new language features (including Vulkan-specific attributes) now fall under the scope of the microsoft/hlsl-specs repository, and access to arbitrary SPIR-V builtin inputs is being considered as part of the Update to inline SPIR-V proposal.