GLSL doesn't use that type. SPIR-V used for a while but later started
relying on its own data structures and stopped using it.
See ca62e849d3 ("nir/spirv: Stop using glsl_type for function types")
If we were ever to add this one again, would be better to have a way to
grab a key for lookup that did not require allocations, right now that's
needed to inject return type as the first element in params array.
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25160>
3890c60584