Atomic ops on images with Unknown type will cause SPIR-V validation failure, and assertion failure in Mesa debug builds. D3D12 allows atomics on typed buffers, and this requires a distinction to be made between UAV reads and atomic ops.
-- v2: vkd3d-shader: Introduce DESCRIPTOR_INFO_FLAG_UAV_ATOMICS and always declare UAV images with known type for atomic ops. tests: Test a typed UAV buffer in test_atomic_instructions().