generator: Apply must_use attributes to all Vulkan structs (#845)

All structs are marked as `Copy`, and builders move `self` for a
convenient builder pattern directly on `default()` (using `&mut`
requires requires first keeping the instance alive in a `let` binding).
This however leads to builder functions accidentally moving a copy of
`self`, mutating it, and dropping the result directly when the caller
did not consume the returned value in ad-hoc field updates, in turn
leaving the author confused why their code compiles without warnings
while the struct was not updated.

Annotating all Vulkan structs with `#[must_use]` should at least give
them an idea why.

Author: MarijnS95

Changelog.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `VK_NV_cuda_kernel_launch` device extension (#805)
 - Added `descriptor_count()` setter on `ash::vk::WriteDescriptorSet` (#809)
 - Added `*_as_c_str()` getters for `c_char` pointers and `c_char` arrays (#831)
+- Added `#[must_use]` to Vulkan structs to make it more clear that they are moved by the builder pattern (#845)
 
 ### Changed