This variable attribute is used in HLSL to add Vulkan specific builtins
in a shader.
The attribute is documented here:
17727e88fd/proposals/0011-inline-spirv.md
Those variable, even if marked as `static` are externally initialized by
the pipeline/driver/GPU. This is handled by moving them to a specific
address space `hlsl_input`, also added by this commit.
The design for input variables in Clang can be found here:
355771361e/proposals/0019-spirv-input-builtin.md
Co-authored-by: Justin Bogner <mail@justinbogner.com>
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