Merge override-expression sized arrays into existing types once resolved

[kentslaney]

Connections
Resolves #6722

Description
When an array size override-expression resolves into an existing type, naga panics because types are in a unique arena. The solution for when this happens is to add a Resolved variant to the PendingArraySize type which is unique by the global expression it is initialized by. The struct also has a handle field for the type it is being merged into. If any of these are used, compact is called again after the pipeline overrides are resolved. Compact then removes those Resolved types and maps the handle index to the type it is merging into.

Testing
This change modifies the existing tests by adding an array with the size of the test array when the size is overridden.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md.

[cwfitzgerald]

wgpu side good after nit

[teoxoy]

I'm not a fan of the new ResolvedArraySize and having to use the compactor in this case but I'm having a hard time coming up with a better solution. I asked @jimblandy for a 2nd opinion.

[kentslaney]

Compact is used twice as it is. This would only add to that total when array size override-expressions overlap.

I'm also not sure what scale we expect this unique type arena to be.

The usage of the compactor makes sense to me in this case because types are being merged and it already has the logic to go through and adjust the handles.

[teoxoy]

@jimblandy and I talked about this and he proposed that we try to not rebuild the types arena and instead make the backends responsible for resolving the array length.

Since I already proposed a few things that didn't work in the end I figured I'd try to make the changes myself. I think it turned out well: #6787.

[kentslaney]

Obsolete due to #6787