[Migrated] Declaring RuntimeArray without `Capability::RuntimeDescriptorArray` set on `SpirvBuilder` emits an unintuitive error message.

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/879
Old labels: t: enhancement
Originally creatd by SiebenCorgie on 2022-06-07T13:28:24Z

When trying to compile a shader with a runtime array descriptor without having Capability::RuntimeDescriptorArray enabled on SpirvBuilder emits an unintuitive error.

For instance the following main function:

#[spirv(fragment)]
pub fn main_fs(
   ...,
   #[spirv(push_constant)] push: &PushConst,
    #[spirv(descriptor_set = 0, binding = 0)] sampled_images: &RuntimeArray<SampledImage<Image!(2D, type=f32, sampled)>>,
    ...
) {
    //...
    let img = unsafe { sampled_images.index(push.texture_indices[0] as usize) };
    //...    
}

emits this error:

error: OpVariable, <id> '19[%sampled_images]', is attempting to create memory for an illegal type, OpTypeRuntimeArray.
       For Vulkan OpTypeRuntimeArray can only appear as the final member of an OpTypeStruct, thus cannot be instantiated via OpVariable
         %sampled_images = OpVariable %_ptr_UniformConstant__runtimearr_40 UniformConstant

While the error is correct without the capability, the use of RuntimeArray probably indicates that RuntimeDescriptorArrays is assumed to be present.

A small hint like: "Do you need to enabled the RuntimeDescriptorArray capability?" would be appropriate,

Empieza reproduciendo el punto de entrada del shader informado, main_fs, con un RuntimeArray, mientras SpirvBuilder carece de Capability::RuntimeDescriptorArray, e inspecciona dónde se produce ese diagnóstico. Se considera completado cuando la capability no compatible se identifica con una indicación clara para habilitar RuntimeDescriptorArray, mientras el contexto existente de tipo no válido sigue estando disponible.